Not to be mistaken with the character Raven, Raven tomes are normal weapons that grant the user a weapon triangle advantage against colorless units. This is extremely desirable as colorless units are outside of the weapon triangle, causing them to take and deal neutral damage. Furthermore, due to healers and archers gaining an ever-increasing viability (such as Bridal Cordelia or Wrathful and Dazzling Staff), the Raven tome is one of the best answers you could bring to the meta.
Due to these tomes giving a triangle advantage against colorless units, the Triangle Adept passive will stack with this tome's effect. Said passive is excellent in both offense and defense as it not only magnifies your attack against gray units due to the triangle advantage but reduces those same foes' attack due to triangle disadvantage. As such, the Triangle Adept passive combined with a Raven tome is rightfully one of the earliest synergies discovered in the Fire Emblem Heroes community.
There are three notable play-styles of Triangle Adept Raven mages. The first being an absolute wall that can slowly wither down their advantage match-up; the second being a retaliation nuker with sky-high offensive prowess that kills anything with the weapon advantage; and the third being a combination of the 2 (which indeed exist!).
The main idea is to be either a nuclear bomb or an ultimate wall against the majority of units in the game, all while serving as a check for colorless units.
Note: With the introduction of Cancel Affinity, Triangle Adept might not be the best skill in certain circumstances. The closest skill that could counter this skill is probably Distant Defense or Fury. Alternatively just be careful with this skill, as those characters did sacrificed their B slot for the skill, which could make them slightly easier to handle (such as Bride Cordelia without Desperation).
Other Skill Choices
Passive Skill B
No questions asked, Breaker skills are the best way to go, and ideally one of the same color as the wielder.
With the combination of a Raven tome and Triangle Adept, the unit becomes a strong answer to colorless and weapon advantage units. Hence, it is unnecessary to use a Breaker skill which counters the weapons in those colors. Since you can't use a Breaker to gain the upper hand against your color disadvantage (for example Hector with SwordBreaker), this meant that a weapon Breaker with the same color as the wielder (Sanaki with SwordBreaker) is the most effective option to use.
SwordBreaker is highly recommended for red mages, and Green TomeBreaker should be used with green mages due to their common sight. As for blue, you could choose either LanceBreaker or Blue TomeBreaker depending on whichever is a bigger threat to your team.
Special skills aren't too much of a factor in a Triangle Adept Raven build and is highly dependent on the character you use for said build. Bonfire/Ignis should be taken for physical walls, Iceberg/Glacies for magical baits, and Draconic Aura/Dragon Fang for the high ATK mages.
Special mentions to Astra for nuking the opponent to oblivion, as well as the weapon magnification effects making the once-in-a-blue-moon 3 digit damage value appear.
This build isn't restricted to a limited amount support skills, so anything goes. Just do not use Shove because its viability is questionable at best.
Passive Skill C
There are again no restrictions, so feel free to load up a buff in order to support your team. Your team does need to answer for the characters that the mage has disadvantages on, after all.
Wall-based mages would prefer the ability to survive (HP+3), while heavy nukers would like offense (ATK+1). Increasing SPD (SPD+1) is not a bad option if your mage is not as slow as Sophia.
The ability to take hits is something that a Triangle Adept Raven mage must have. Hence, having decent bulk both magically and physically is a very desirable trait. A decent ATK stat is equally desirable since you will still need to kill things, and a wall that cannot kill a target is just... a wall (We're looking at you, Henry).
Note that the list below is not meant to be exhaustive.
These are some mages that have a decent ATK stat so they will be able to kill characters they have an advantage on rather effectively. They also sport an overall decent defensive bulk which allows them to take any punishment thrown from their advantage match-ups.
These mages have an extremely high ATK power which means that an attack with a weapon advantage will nuke almost any character out of existence. The defensive properties of the Triangle Adept Raven set will also increase their damage tolerance. They will not be walling anytime soon, but they will at least be able to hold their ground if they are under attack.
Sophia is the prime example of a perfectly compatible mage with the Triangle Adept Raven build.
The Nabata Prophet is a rare breed in the sense that she has three very respectable stats (ATK, DEF and RES). This makes her very good in terms of being both a wall and a nuke. Her slow SPD is a non-issue since everything that Sophia has an advantage on will hit her for single digits. Hence, Sophia is a specimen able to unlock the true potential of the Triangle Adept Raven build.