Múspell

Weapon: Flamelick Breath

Assist: Rally Up Atk+ / Other Rally+ Variations

Special: Glimmer / Bonfire

Passive A: Atk/Def Catch / Atk/Res Catch / AR-D Def/Res 4 / Sturdy Stance / Mirror Stance / Life and Death 4 (from Selena) / Flexible Enemy Phase skills

Passive B: Def/Res Ruse / Atk/Res Ruse / Other Ruse Variations

Passive C: Domain of Flame

Sacred Seal: Atk Feint / Res Feint / Distant Guard / Close Guard / Drive Atk / Various "Within Two Spaces" or Feint skills

• Another active form of support that Múspell can contribute is the classic Rally + Ruse skill combination, allowing him to manually apply various stat penalties as well as【Guard】in rows or columns of your choice. This has several advantages, including:
  • A more proactive role on Aether Raids Defense as the Ruse penalties are fairly easy to spread out on Offense players anticipating combat in certain columns. Constantly Rallying with dance support also means Múspell is less likely to self-destruct on the enemy, instead prioritizing boosting his teammate's stats.
  • The Rally stat boost and Ruse penalties can really roll the ball in your team's favor. Any Ruse involving Res, for example, makes it much easier for your units to utilize Domain of Flame's huge damage boosts as it compares the difference in their Atk minus the enemy's Res. 
  • 【Guard】also delays Special activation, making Special trigger-reliant threats such as Fallen Edelgard (who would retaliate with a powerful Bonfire under normal circumstances) significantly more manageable to approach.
• As far as the actual skills themselves:
  • Múspell's default Rally Up Atk+ will definitely suffice in many situations, boosting the damage of surrounding allies rather than just one. If you'd like to boost more stats or have Múspell continue Rallying other allies on Aether Raids Defense, it's best to replace it with a single-target Rally.
  • As mentioned above, a Ruse that affects Res specifically is sought out to increase damage from your units' magic attacks as well as, more importantly, Domain of Flame's true damage. Lacking those, you can simply carry on with other Ruse variations and even run the Res Feint Sacred Seal to compensate if you'd like. 
  • While the build runs best with a Feint skill to penalize three enemy stats as mentioned briefly, you can also just run supportive "within two spaces" skills in Múspell's Sacred Seal as you'll still want your units to park near him for Domain of Flame's boosts. An easy one to recommend in this case would be Drive Atk.
• The rest of the build doesn't matter much compared to the previously mentioned slots. If you're able to provide them, however, a Catch skill works splendidly as they work with the penalties Múspell provides with his chosen Ruse.  
  • As a somewhat weird suggestion, if you're grabbing Atk/Res Ruse from Selena (FE8) in particular, you can also grab Life and Death from her as a sufficient (if not a little meme-y) offense boost option. The added Speed from both Flamelick Breath and Life and Death will serve to boost Múspell's paltry Speed to a more effectively acceptable 28. 

Weapon: Flamelick Breath

Assist: Assistless / Rally Up Atk+

Special: Glimmer / Bonfire

Passive A: Atk/Def Unity / Sturdy Stance 3 / Bracing Stance 3 / AR-D Atk/Def / AR-D Atk/Res / Death Blow / AR-D Def/Res 

Passive B: A/D Near Trace / Dragon Wall

Passive C: Domain of Flame

Sacred Seal: Distant Guard / Close Guard / Drive Atk / Various Spur Equivalents

• Múspell can balance his ability to grant power to fellow allies while keeping his combat utility intact. Keep in mind, however, that lacking good initiation options like high Speed, guaranteed follow-ups, or Impact skills may make him underperform a bit as an attacking unit; much of his utility instead stems from his ability and power up nearby allies in early-turn formations with Domain of Flame. . 
• Due to the joint supportive nature of Domain of Flame, it's generally a good idea to place priority around utilizing it rather than prime Múspell's combat. 
  • In addition to providing a basically free Atk/Def +4 to both Múspell and his surrounding defenders, Domain of Flame gives another potential form of damage: additional true based off of Múspell / an ally's Atk minus the enemy's Resistance. Provided you account for the second effect and build your formation so all defenders' Attack levels are fantastically high, this should prove to be a useful way to boost damage and physical bulk all around the team.
  • Another "within two spaces" skill in the Sacred Seal further contributes to Múspell's support utility. The easiest to recommend here is Drive Atk as it further enables Domain of Flame's true damage based on his ally's Atk minus the opponent's Resistance differential (translation: bing bong Drive Atk 10 hit hard). Close Guard or Distant Guard are secondly great they cover an allies' Defense and Resistance in melee or ranged combat, respectively. 
• Flamelick Breath is pretty much a keeper and one of the other reasons you're running Múspell on your Aether Keep; the 【Deep Wounds】 penalty can be a gamechanger, especially when it's in weapon form and thus basically unblockable. It follows that Múspell's survival typically matters very little as the penalty applies as long as he gets in one attack. Needless to say, enemy superunits who rely on sustain to stick around will not appreciate the added healing obstruction.  
• For general use and for Aether Raids Offense, you can't really go wrong with Reposition or any other positional skill of choice to put your team members in more ideal spaces. Going without an Assist skill is generally a good call in AR-D if you need to prioritize other allies moving or using their Rally assist before Múspell does. A more aggressive formation involving Múspell going first and spreading 【Deep Wounds】, however, may require Múspell to use his native Rally Up Atk+ so he rallies and gets Danced first.
• A lack of Impact skills leaves Múspell a bit lost for actually useful A-slots, so it's generally best to seek skills that increase Múspell's initiation damage, bulk, or perhaps both. To prevent opposing Galeforce offense units from charging said special, you may also consider running a high-tier Stance skill as they come built with cooldown delay. 
• The average layperson need not think too much about Múspell's B slot passive outside of his native Dragon Wall; its Resistance-based damage reduction is usually enough to delay Múspell's demise against all but the most Res-heavy units he initiates against. If you're able to access A/D Near Trace, however, its useful Canto effect (movement after combat) helps Múspell seek more favorable spaces after attacking and surviving, and the extra raw Attack debuff is always nifty. 
• As a usage tip, if you have Bridal Catria/Thea available in your barracks, it's recommended to run Múspell with them on Aether Raids Defense as the duo can apply the【Triangle Attack】effect to him. This allows Múspell to attack twice (when other 【Triangle Attack】induced allies are near him) and thus overcome his otherwise unexceptional Player phase performance. Overall, this combination serves to significantly increase Múspell's overall offensive presence and serve as an unexpected initiation threat. 

Weapon: Flamelick Breath / Pale Breath+ (+Def) / Blackfire Breath+ (+Def)

Assist: Positional Assist

Special: Glimmer / Bonfire

Passive A: Atk/Def Unity / Close Def 4 / Atk/Def Catch / Sturdy Stance / Atk/Def Bond 

Passive B: Dragon Wall / A/D Near Trace / Dragon's Ire

Passive C: Domain of Flame

Sacred Seal: Quick Riposte / Atk/Def Solo / Sturdy Stance

• When utilized alongside an armored carry unit utilizing a Far Save skill, Múspell can run a very effective variant of the previous build that's specifically focused on investing in melee combat performance. While his fellow Far Save ally handles all ranged combat, Múspell can utilize Domain of Flame to significantly strengthen said ally as well as himself. 
• As a side note, Múspell's Resistance is also great enough that stray dragon attacks aren't a particularly high concern (don't get cocky versus Seiros, though) and he can make typically full mileage out of Dragon Wall's damage reduction against most common Defense attackers.
• The main change here is the transition from Distant Counter to a stats-boosting A-slot passive skill, allowing Múspell to prime himself better for raw combat rather than ranged counterattacking. 
  • Atk/Def Unity synergizes well with the nearness required of Múspell's Far Save ally and provides protection versus penalties. 
  • Close Def doesn't improve Múspell's raw damage but prepares him better versus dragonstone enemies while providing a pseudo-Lull skill effect (that is, cancelling enemy buffs). 
  • Other Atk/Def skill equivalents are up in the air if you can access them. The most notable are Atk/Def Catch — which technically provides one of the highest Atk/Def boosts of +9 if you abide by both of its conditions — and Sturdy Stance 3 — whose Guard effect will always be handy versus enemies with short charged Specials. 
• To ensure a greater chance of killing melee enemies in one sitting, a follow-up skill is required one way or another. This is done through Quick Riposte in the Sacred Seal, or the more effective and longer-lasting Dragon's Ire if you're willing to make a fair trade for the utility of other B-slot skills. 
• Speaking of B-slots, A/D Near Trace is a third alternative to Dragon Wall and Dragon's Ire before it. It gets some nice usage in combination with Flamelick Breath's 【Deep Wounds】penalty spread as Múspell can initiate after Bolt Tower activates (thus disabling the enemy defenders from healing it back with their Healing Tower) and retreat back to the safety of his Far Save ally after combat. Use this only if you're confident in your ability to position both units before and after the fact. 
• If you're not a fan of Flamelick Breath's limited longevity (that is, it's only really effective if Múspell keeps above 25% HP), Pale Breath and Blackfire Breath are great inheritable options to consider. Both provide technically superior combat boosts (+5 HP and +4 to whichever defense you choose, preferably Defense) while having no HP-related caveats. Note that Blackfire Breath requires Múspell to stay a space away from his Far Save ally, which is still favorable and not particularly difficult to achieve, but can be rather inconvenient in strained situations. 
• Bonfire is a pretty great Special under the assumption Muspell will get doubled by most enemies he comes across and follows up with Quick Riposte or Dragon's Ire. If you have some source of cooldown acceleration (like Summer Hilda or New Year Velouria) for instant 2 CD retaliation, or simply don't want to take chances with timing, Glimmer is a short and reliable option. 

Weapon: Flamelick Breath / Blackfire Breath+ (+Res) \

Assist: Positional Assist

Special: Bonfire / Sol / Iceberg / Glimmer / Noontime

Passive A: Distant Counter / Distant Ward

Passive B: Dragon Wall / Dragon's Ire

Passive C: Domain of Flame

Sacred Seal: Quick Riposte / Atk/Res Solo / Mirror Stance / Mystic Boost / Atk/Def Solo / Stance equivalents of listed skills

• Múspell can easily play the part of a dedicated tank (especially in Aether Raids Offense) thanks to his fantastic distribution in all categories but Speed. Múspell will be doubled frequently, however, meaning a lot of emphases will be placed on his single-shot damage and ability to take multiple rounds of hits. 
• Much of Múspell's base skills fortunately already fulfill much of the build's goals:
  • Dragon Wall scales well off of Múspell's base Resistance on top of additional investment through skills and Mythic blessings. In most cases, Múspell will probably fulfill its maximum conditions, granting up to 40% total damage reduction and practically making all but the most powerful effective weapons bounce off of him.
  • Domain of Flame grants additional forms of damage through Atk/Def +4 and further true damage based on his Attack minus the enemy's Resistance. Assuming Múspell isn't facing off a total magic sponge, this part of the skill should hold true for the majority of common targets, giving Múspell a self-sufficient source of damage (excluding any additional support he's getting from other allies!).
• Awkward as it may be to totally replace Muspell's unique Breath weapon, Blackfire Breath is worth a very considerable amount of thought. While Flamelick Breath boasts better raw Attack and Fatal Smoke built-in, a lot of its effectiveness is lost upon going below 25% HP, and besides this, its【Deep Wounds】 isn't particularly urgent to have on an Aether Raids Offense tank.
  • Meanwhile, in exchange for just some Attack, Blackfire Breath's stat swings and enemy bonus negation are consistent at any health level as long as Múspell is separated from his allies. Thanks to Múspell's enhanced movement, it is easy in practice to abide by this while benefiting from ally skills like Drive Atk or Distant Guard from a space away.
  • Additionally, Blackfire Breath is refineable, meaning Múspell technically gains superior defensive performance thanks to increased +5 HP and a defensive refine. With a +RES refine and Blackfire Breath's built-in Lull skill (the enemy's Attack and Resistance take a pretty deep dive of -5), it synergizes almost superiorly well with Dragon Wall, too. 
• Given how rare Blackfire Breath is, though (it's a seasonal weapon on fellow red Dragon Halloween Dheginsea), Flamelick Breath is a perfectly doable base tool that bolsters Múspell's statline in all categories +5 except HP — the Fatal Smoke and +3 Atk on top of 16 Might are also handy to disable any sources of enemy healing (especially Healing Tower if Múspell initiates into a group of enemies with ideal positioning) as well being his best raw damage-dealing option. 
• The only urgent inheritance needed here is Distant Counter, a basic requirement for any sole omnitank to maximize counterattacking options. 
  • Though far less recommended than normal Distant Counter, Distant Ward is worth looking into if making Múspell a dedicated magic tank with a secondary Near Save ally seems is a priority strategy for you.   
• Múspell's natural inability to make follow-up attacks takes up a few considerations:
  • You can run Dragon's Ire in the B slot or Quick Riposte as the Sacred Seal to create follow-up opportunities for Múspell and lower the possibility of enemy survival due to two attacks. Running either comes at the cost of some rather nifty skills in those slots, though, and you'll probably need to provide equivalent effects through other allies' support skills (like Flayn's Caduceus Staff). 
  • Lacking an immediate method of cooldown acceleration and generally being guaranteed to get doubled means Bonfire (or Iceberg if Múspell's Resistance is at a higher number) or Sol are Múspell's best Special choices due to reasonable cooldown and activation returns. 
    • If you DO happen to have some special form of Special cooldown charging like Summer Hilda's Sunshade Staff, however, Glimmer and Noontime are immediately preferred for instant activation on every counterattack. 
• As far as seals are concerned, Atk/Res Solo has been mentioned shortly before as a great damage and magical resilience padder, and it pairs well with the Resistance-scaling Dragon Wall and the also-Solo-activated Blackfire Breath. You may run a variety of similar alternatives to it, or to Atk/Def Solo if you prefer more physical resilience. Without a healing Special, Mystic Boost is also worth mentioning for some form of sustain, though a properly-built Offense team can possibly mean Múspell won't need it the majority of the time