Fire Emblem Heroes Black Knight How to Get
Black Luna
The Black Knight's exclusive special is one of the main things that make him unique among sword armors, letting him cut off 80% of the foe's Defense when it triggers.. This makes the Black Knight's special triggers deadly should the foe lack damage reduction such as Spurn.
Refined Alondite
The Black Knight's long awaited refine is finally here, and it drastically improves him thanks to the simple addition of the Slaying effect, with a stat boost as the cherry on top. Thanks to Alondite's newfound Slaying, Black Luna is reduced to a base 2 cooldown, and with the addition of Special Fighter or a Breath Skill, can instantly trigger it in retaliation to an attack if the foe lacks Guard, making the Black Knight dangerous to initiate against.
Terrible Resistance
The Black Knight's poor Resistance is his greatest weakness, making him easy prey for Player Phase mages such as Lysithea in addition to dragons that can take a hit from Black Luna such as Fallen Male Corrin and Seiros.
Effective Damage
While the Black Knight benefits greatly from being an Armored unit, he also gains vulnerability to effective weaponry such as the Slaying Spear and Micaiah's Thani, which can damage him heavily even if they fail to KO him.
BK's fun at +10! My first completed grail merge project. Standard Alondite and Black Luna, since they're both great already, Swap because he's generally used on my armor emblem team. The build mostly explains itself- at +10, BK's got enough speed to hold his own against a lot of matchups, so Special Fighter is used for consistent BL activation. Ward Armor since, as said before, I use him on my armor team, finishing off with Steady Posture in the seal slot to grant him an extra boost in enemy phase, since I like chucking him into the middle of the map and watching.
Show Explanation/Analysis 
Alondite A Steady Breath 
Swap B Special Fighter 3 
Black Luna C Ward Armor S Steady Posture 2
Black Knight build by Steak 02-23
You don't take physical damage and are able to double enemy phase and prox Black Luna for the kill. BK can tank one non-armor-effective hit with no bonus (also excluding specials). Can kill Dimitri and each Edelgard since his stats go past most builds defense and speed. Now if a situation comes you cant damage a physical damage unit you just let them attack you, they won't do much and you can build your special which should at least come close to killing them. The only problem that this unit has like all other BKs is that it cant tank armor effectiveness hits from mages. Can handle just about every dragon (except Seiros) and the same thing goes for beasts. This is a very all-around build that is easily implementable and good in just about every game mode.
Show Explanation/Analysis Alondite A Steady Posture 3 Reposition B Special Fighter 3 Black Luna C Threat. Atk/Spd 3 S Steady Posture 2
I bought him because when I just came back I fell in love with the game with all it's renewed options and recalled how great he was supposed to be, and how amazing it would have been if I'd gotten one of the CC/DC chara's at first but... he just dissapointed me at every turn.
Show Explanation/Analysis Alondite A Steady Stance 3 Swap B Wings of Mercy 3 Black Luna C Threaten Atk 3
He'll likely be able to kill with the right skills and his Black Luna, but with his base kit he's just meh.
Weapon: Alondite Assist: Swap / Pivot Special: Black Luna Passive A: Steady Breath Passive B: Bold Fighter 3 / Special Spiral 3 / Special Fighter 3 Passive C: Threat. Atk/Def 3 Sacred Seal: Brazen Atk/Spd 3 / Quick Reposte 3 This set aims to make Black Knight a speedy physical tank with high damage potential and good speed to nullify most follow up attacks against him and maximize his own follow ups. Alondite and Black Luna are self explanatory, distant counter capabilities with arguably the best special in the game. The combination of Steady Breath and Bold Fighter 3 makes him able to charge his special by +2 through both phases making Black Luna practically a guarantee through all match ups. The combination of Threat. Atk/Def 3 and Brazen Atk/Spd 3 is there to raise all his key stats to Atk 62, Def 44, Spd 41 under ideal situations. Now for budget/alternative skills, both Specials (B Skill) will still give him a quicker way of using Black Luna but at the cost of a guaranteed follow up attack while initiating. Spiral would help proc Black Luna almost every battle while Fighter allows him to have +1 charge per attack while giving a -1 charge adding to his survivability. If you're going for the budget/alternate skills I'd highly recommend going with these combinations of: Quick Reposte 3 and Special Fighter 3. Brazen Atk/Spd 3 and Special Spiral 3 (This is the alternate setup I'd recommend)
Show Explanation/Analysis Alondite A Steady Breath Pivot B Bold Fighter 3 Black Luna C Threat. Atk/Def 3 S Brazen Atk/Spd 3
Now onto the actual explanation.
Steady Breath has the added benefit of Def +4 making him have a Def of 44 under ideal situations.
Sadly Threat. Atk/Def 3 will weaken Black Luna due to the -5 Def but, with the +12 Atk from itself and Brazen Atk/Spd 3 more than makes up for it.
The Spd +7 puts his Spd at a sweet spot in most cases, it's fast enough to keep Spd built units from following up on him and allows him to follow up on most average units.
In conjunction with Bold Fighter 3, during player phase he would be able to follow up everything while not being followed up by most.
During the enemy phase he would still be able to keep most Spd built units from following up on him but lose the ability to follow up on them but against average units he would be able to keep his follow up due to the sweet spot his Spd stat would be at.
Quick Reposte 3 as his seal would work really well with Bold Fighter 3 as it would guarantee follow ups on both phases but at the cost of having to be ≥80% Hp and once you dip below that it becomes useless while Brazen Atk/Spd 3's ≤80% Hp threshold is just as easy to get to while having a longer window of viability.
Losing Brazen Atk/Spd 3 will keep his Spd at 35 meaning anything with 40 Spd or more will be able to double him and Special Fighter 3 would help mitigate any specials from procing on him.
Unlike the other alternate setup this would keep his ability to not be followed up easily and after the first battle almost guarantee Black Luna every match.
During enemy phase Black Luna is guaranteed, during player phase it is only guaranteed against average Spd units as with a Spd of 41 Black Knight should be able to follow up.
Black Knight build by MagmaHex 02-29
If he's the only armor on your team, use Panic Ploy instead of Fortify Armor. Instead of Wrath, you can also run Special Fighter. His seal can be Quickened Pulse, If you want his special even faster. I don't use that build atm, because no grails, but it'll be the build I'm gonna use. I don't want to use Bold Fighter, because, first, I have no fodder, second, I don't like the mixed phase thing. I like using Wrath in general, because 10 true damage, if I don't use Desperation or Vantage on the unit. The build costs 2080 SP, if used with Panic Ploy, and 2010 SP, if used with Fortify Armor. Side Note: My Team is standard Altina, Default Nino (Fury+Desperation), Standard Peony and V!Faye (with Armor March)
Show Explanation/Analysis Alondite A Steady Breath Reposition B Wrath 3 Black Luna C Fortify Armor S Chill Def 3
Fire Emblem Heroes Black Knight How to Get
Source: https://gamepress.gg/feheroes/hero/black-knight