Masked Marth

Weapon: Sealed Falchion (+Eff)

Assist: Positional Assist

Special: Moonbow / Luna / Bonfire

Passive A: Fury

Passive B: Desperation

Passive C: Rouse Spd/Def / Flexible

Sacred Seal: Brazen Atk/Spd / Swift Sparrow

• If you're not willing to put in heavy investment for Masked Marth just yet, the good ol' FuryDesp combination, fortunately, works extremely well with both his statline and the effects of refined Sealed Falchion. As a result of being cheaply built and honoring both of Sealed Falchion's conditions, Marth remains a very respectable offensive threat for general content.
• Fury is cheaply available in the 3-4 star pool and provides a nice amount of stat padding across all of Masked Marth's stats. Additionally, the recoil effect suffered after combat synergizes well with:
  • Desperation, allowing Marth to make two attacks in a row provided he can make a follow-up.
  • The base effect of Sealed Falchion, which provides a hefty boost of +5 to all stats if Marth is damaged at all (or has a visible bonus, which is also good but not always guaranteed in all scenarios).
  • The Brazen Atk/Spd Sacred Seal, which also offers a respectable boost of +7 to Marth's Atk/Spd if he lands conveniently in Desperation's HP threshold. For initial reliability, you may opt for Swift Sparrow instead.
• Moonbow or Luna are pretty standard Specials which allow Marth to bypass high Defense enemies in a pinch. Bonfire is technically pretty reliable as Sealed Falchion can bring Marth's Defense up to an effective 35+ (38 with Fury) under perfect conditions, giving a solid burst damage of 17-19 or more.
• The rest of the build is mostly fringe and personal preference. The most affordable C slot that works with Masked Marth's weapon is Rouse Spd/Def, which grants self-provided bonuses to himself provided he isn't adjacent to any allies.

Weapon: Sealed Falchion (+Eff)

Assist: Positional Assist

Special: Noontime / Sol

Passive A: Distant Counter / Atk/Spd Solo

Passive B: Spurn / Close Call / Repel

Passive C: Atk Smoke / Pulse Smoke

Sacred Seal: Spd/Res Solo / Kestrel Stance

• This build is pretty much traditional on high-Spd, high-Atk infantry units like Masked Marth at this point. That is, to compensate for Marth's otherwise forgettable base bulk even after Sealed Falchion's effective +10 to relevant stats (Res only misses out at +5), running Repel or Close Call or Spurn in the B slot allows him to take advantage of his fantastic resulting Speed instead to bear the force of incoming attacks.
  • Running any of the three aforementioned skills is important for this build due to their Speed-based damage reduction effect—4% less damage for every Speed point compared to the enemy unit for up to 40%. After the fact, Marth becomes a formidable superunit, especially considering Sealed Falchion can prevent the foe's follow-ups, which further compounds his tankability especially against units with guaranteed follow-up attacks such as Bramimond.
• There's little else better than Distant Counter for Marth's A-slot as it allows him to counterattack ranged units. This is especially in Aether Raids Offense as it prevents Marth from being whittled down too easily by the multitudes of ranged enemies in that mode. Lacking that or simply wanting to make Marth a melee-only, non-AR unit, Atk/Spd Solo is a great alternative as it further improves Marth's hitting power and damage reduction in both player and enemy phase.
• Spd/Res Solo or Kestrel Stance are Marth's best Sacred Seal choices for this build, as both provide more Speed to make better use of Spurn/Close Call/Repel while improving his bulk or damage output, respectively.  
• Atk Smoke tends to work consistently on Masked Marth given it further debilitates incoming damage from surrounding foes while being easy to access. Pulse Smoke is also a good option, however, as it badly cripples enemy setups reliant on precharged units.
• Finally, to strengthen the purpose of this build and significantly increase Marth's longevity, a sustain Special will be preferred above anything else. Noontime is reliable due to its low CD of 2 (and the fact that it activates on every counterattack with Geirskögul support), whereas Sol has higher healing returns at a more exploitable CD of 3.

Weapon: Sealed Falchion (+Eff)

Assist: Positional Assist

Special: Galeforce / Bonfire / Luna / Ruptured Sky / Moonbow

Passive A: Swift Sparrow 3 / Atk/Spd Push 4 / Atk/Spd Solo 4 / Fury / Flashing Blade

Passive B: Desperation / Wings of Mercy / Lull Spd/Def Lull Atk/Spd

Passive C: Time's Pulse

Sacred Seal: Flashing Blade / Heavy Blade / Swift Sparrow / Brazen Atk/Spd

• This is a high-investment version of the previous offensive build, with elevated priority on Special activation thanks to Time's Pulse precharging Marth's chosen Special by -1 at full CD count. Throughout all versions of this build, Sealed Falchion still makes Masked Marth as powerful as ever due to largely increased stats (+10 to all) and ability to stop the enemy's follow-up in a pinch. Several possible builds from this are as follows:
  • Galeforce:
    • To activate Galeforce in a single round, Flashing or Heavy Blade in some form is also necessary to circumvent the still-long cooldown count of 4 after Time's Pulse. This can be done through Marth's A slot, Sacred Seal, or providing Infantry Flash or Infantry Rush support nearby.
    • After getting into the proper HP threshold, Desperation makes it relatively easy to trigger in two successful hits without the need for a counterattack, though a relevant Lull skill may make it easier to get Flashing / Heavy Blade going successfully.
    • On an Aether Raids Offense team focused on chaining Galeforce, running Wings of Mercy would allow her to get into the thick of the action more conveniently, provided you have a unit to act as the beacon. Furthermore, if you're not confident in Marth's ability to get two shots off and activating his Special (since one shot kills are possible), you can opt to run ally-provided precharge support, especially Velouria's Wolfpup Fang to circumvent this.
  • General Offensive:
    • Running Luna or Bonfire generally requires Flashing / Heavy Blade in some form as well, if you'd like to activate either of these in a single round of combat.
    • If you'd like to focus on overwhelming the enemy with extremely high stats, however, you can simply run stat-boosting skills in both Marth's A-slot and Sacred Seal in conjunction with either Ruptured Sky or Moonbow. After Time's Pulse, these two Specials only require one action to charge, meaning no Blade skill is required at all and you can focus on running stat seals like Brazen Atk/Spd or Swift Sparrow.
    • Most Atk/Spd-boosting A slot choice will do well here, though Fury or Atk/Spd Push 4 are especially notable for inflicting recoil damage and fulfilling an essential effect of Sealed Falchion.
• There's little better than the good and ol' reliable Desperation, which synergizes amazingly with the prior damage effect of Sealed Falchion while making Marth simply harder to stop in more general gameplay modes. Sealed Falchion even provides an partial form of Null Follow-Up, making that largely unnecessary for consideration on this build.