Yamask can be found on 1F and B1F of Relic Castle, with an encounter rate of 50%. Their levels range from 19 to 22. Cofagrigus can be found on B2F until B7F of Relic Castle. From B2F to B6F, their levels range from 34 to 37, and their encounter rate is 50%. On B7F, their encounter rate is 20%, and their levels vary from 48 to 50.
Even though Yamask isn't exactly the most commonly seen Pokémon in Nuzlockes, coming to Relic Castle gives you a 50% chance of catching one. Yamask itself is nothing special. It has good physical bulk, but its other stats are mediocre to bad. This changes as soon as it evolves, though. Cofagrigus is an absolute defensive monster, and its Special Attack isn't exactly bad either. The only downsides are its low Speed and HP stats, but this doesn't stop Cofagrigus from being one of the best mixed tanks in BW's regional Pokédex. Plus, its typing also helps it with its defensive role, only being weak to Ghost- and Dark-type attacks. And almost any Pokémon that doesn't possess a move of these types is a match for Cofagrigus.
N (Nimbasa City): Don't use Yamask. Just don't. Scraggy and Sandile have Faint Attack and Assurance, respectively, which will hit Yamask for super effective damage. Darumaka can only hit Yamask with Fire Punch, but due to it being powered up by Hustle, Yamask will take quite some damage, even with its high Defense. Plus, Yamask has no way to hit back, and Will-O-Wisp doesn't affect Fire-types. Sigilyph's special attacks are just too powerful for Yamask's mediocre Special Defense.
Gym #4 - Elesa (Nimbasa City, Electric-type): Risky. Elesa's two Emolga both know Volt Switch off a decent Special Attack stat, and they will hit Yamask hard. Switching out is a bad idea as well, as they both have Pursuit. Once these are out of the picture, Yamask could use Will-O-Wisp against Zebstrika to cripple its physical attacks, but Zebstrika also knows Volt Switch, so it's preferred to just let Yamask use Will-O-Wisp, and then switch it out.
Cheren (Route 5): Liepard just hits too hard with Pursuit. Stay away. Pignite and Pansear can't be burned, they are bad matchups as well. Dewott and Panpour know Water Pulse and Scald, respectively, which makes them too risky to battle. Servine and Pansage both know Leech Seed, so they're bad choices as well. The only possible matchup for Yamask is against Tranquill. However, it does have Air Cutter and Roost, so a Rock- or Electric-type is preferred instead of Yamask.
Gym #5 - Clay (Driftveil City, Ground-type): If you have evolved your Yamask at this point, great! That will certainly come in handy! If not, then you may be able to find some usage too. Krokorok's Crunch makes it an obvious bad choice, even for Cofagrigus. Palpitoad is manageable, but only for Cofagrigus. Muddy Water can hit hard, but Cofagrigus's combination of Will-O-Wisp and Hex hits harder. Excadrill is, ironically, the easiest for the two to take. Even Yamask can manage it, although it's preferred to switch it out as soon as it can get Will-O-Wisp in. The reason is that Excadrill gets crippled if it's burned, and Yamask/Cofagrigus can do exactly that.
Bianca (Driftveil City): Ergh. If you haven't evolved Yamask into Cofagrigus at this point, don't bother. Herdier packs the super effective Crunch, Pansear and Pignite are immune to Will-O-Wisp and Dewott, Servine, Panpour and Pansage are all able to destroy Yamask. The elemental monkeys both know Bite, and Pansage knows Seed Bomb and Leech Seed, while Panpour knows Scald. Dewott and Servine will actually like you burning them, as their Torrent and Overgrow will power them up. Avoid doing that. Actually, just avoid battling them with Yamask. Then, there's Musharna. Ominous Wind or Hex are able to hit it, but Musharna hits back equally hard with Psybeam, and is able to use Hypnosis. Try to avoid. Cofagrigus does a lot better than Yamask, and is able to take Musharna and the elemental monkeys.
N (Chargestone Cave): Have you evolved Yamask now? Actually, it doesn't really matter. Even Yamask can fare well here. Will-O-Wisp and Ominous Wind shred his full team without too much difficulty.
Gym #6 - Skyla (Mistralton City, Flying-type): Apparently, Skyla thought it was a good idea to create a Swoobat that only carries physical moves. I don't think anyone has got an idea why. Anyway, you should be evolved by now. Cofagrigus deals with Swoobat incredibly quick. It doesn't even need Will-O-Wisp, as Ominous Wind or Hex just destroys it. Unfezant and Swanna are both special based. Cofagrigus's Ghost-type moves don't affect Unfezant, and it mostly carries special moves, so even burning it isn't necessary. Keep Cofagrigus away. Swanna is also mostly special based. You could use Will-O-Wisp to cripple its Aerial Ace, but honestly, your best bet is to leave Swanna to a Pokémon that can OHKO it with an Electric-type attack.
Cheren (Route 7): Liepard has the Dark-type move Assurance, but honestly, Cofagrigus easily shrugs off that hit before burning it with Will-O-Wisp. The hardest will be to actually hit Liepard, but Hex or Ominous Wind could do the trick. Unfezant will hit hard with Air Slash, and it holds Scope Lens as well. Don't battle it. Simisear and Simipour will prove to be too powerful with their respective Flame Burst and Scald, but Simisage won't like Cofagrigus. Will-O-Wisp kind of destroys it, and Cofagrigus can easily finish it off. Servine is annoying with Leech Seed, stay away. Dewott and Pignite are both possible matchups, as long as you don't use Will-O-Wisp, as that will only boost their respective Water- and Fire-type moves.
Gym #7 - Brycen (Icirrus City, Ice-type): Brycen's Pokémon are best handled by any Fire- or Fighting-type you have at this point, but if you don't have those, don't worry. Cofagrigus is here to help. Poor Vanillish can't really hit hard, and Cofagrigus takes it out with Ominous Wind or Hex, maybe Shadow Ball, if you have it already. Cryogonal has great Special Defense, but bad Defense, so please, handle it with a physical attacker instead of Cofagrigus. It's almost impossible you haven't got one at this point. Beartic's Icicle Crash gets crippled by Will-O-Wisp, before Cofagrigus decides to finish it off with Hex or Shadow Ball.
Bianca (Route 8): Bianca again. Stoutland will land a hit with Crunch, but even though it's super effective, Cofagrigus can shrug it off, even if it crits. Will-O-Wisp proceeds to cripple Stoutland's offensive capabilities. Switch to a Pokémon that can actually hit Stoutland after that, and preferably heal Cofagrigus as well. Simipour's Scald will still be too dangerous. Simisear and Simisage both go down to Shadow Ball. In Simisage's case, you could try Will-O-Wisp as well. Musharna does not like Shadow Ball either, and gets 2HKOd. Watch out for Hypnosis. Emboar is dangerous, but not impossible. Neither Pokémon will hit too hard, though. Serperior can and will set up, don't battle it. Will-O-Wisp won't work either, thanks to Overgrow. Samurott is very possible, as it only has Aqua Jet to touch it. Don't use Will-O-Wisp and finish it off with Shadow Ball.
Gym #8 - Iris/Drayden (Opelucid City, Dragon-type): Drayden and Iris will hate you for having Cofagrigus. Literally all of their Pokémon only have physical moves, which Cofagrigus utterly cripples with Will-O-Wisp. Hope you have a female one in Black and a male one in White, as both Iris's and Drayden's Fraxure have the Ability Rivalry. But even then, a super effective Assurance won't hurt. Druddigon is risky, due to it having Night Slash. Normally it shouldn't pose a problem, though, and Will-O-Wisp will burn it. Haxorus likes setting up Dragon Dance, but Cofagrigus laughs at that before burning it. Haxorus can get a hit in with Assurance, but that wouldn't do nearly enough. Heal when necessary.
Cheren (Route 10): Liepard now carries Night Slash, which hurts, even on Cofagrigus's incredible Defense stat. Try to stay away. Unfezant is still a big no. Avoid all the fully evolved starters. Serperior has been explained with Bianca already. A Coil set-up just isn't fun. Samurott now packs Aqua Tail, and you can't burn it. Emboar can use Flamethrower, which hits Cofagrigus's weaker Special Defense stat. Stay away from Simipour too. Simisear and Simisage are quite easy targets for Shadow Ball and, in Simisage's case, Will-O-Wisp.
Elite Four Shauntal (Pokémon League, Ghost-type): If you're at a higher level than Shauntal's Cofagrigus, then go nuts. But whenever a Shadow Ball lowers your Special Defense, get out. You do not want a crit. Don't battle Jellicent or Chandelure. Their special Shadow Balls will be too powerful for Cofagrigus. Golurk is dangerous. You could be able to burn it, but it knows the Ghost-type move Shadow Punch, boosted by Iron Fist. It's not recommended to battle it, especially since it knows Curse as well.
Elite Four Marshal (Pokémon League, Fighting-type): Throh has Guts, don't battle it. Will-O-Wisp all the other Pokémon, and sweep. Conkeldurr doesn't have Guts, so be sure to take it out. On Mienshao, it's preferred to start out with a Rock- or Dark-type Pokémon, then switch to Cofagrigus, so that Mienshao loses a lot of health, due to Jump Kick not affecting Cofagrigus.
Elite Four Grimsley (Pokémon League, Dark-type): This is the first major battle since Cofagrigus's evolution where it literally doesn't stand a chance. All of Grimsley's Pokémon know either Night Slash or Crunch, and it's all off STAB. Just don't.
Elite Four Caitlin (Pokémon League, Psychic-type): Psychic-types may be weak against Ghost, but Musharna, Sigilyph and Gothitelle all know Shadow Ball, and are faster. Stay away. Reuniclus doesn't, take it out with Shadow Ball.
Reshiram/Zekrom (N's Castle): Zekrom can get burned by Will-O-Wisp, Reshiram doesn't. Zekrom is mostly physically based, Reshiram isn't. That means Cofagrigus can take Zekrom easily, but should avoid batting Reshiram at all costs.
N (N's Castle): The same still counts. Zekrom gets beaten by Cofagrigus, Reshiram should be avoided. Zoroark is an obvious bad choice, as it can use STAB Night Slash. Carracosta and Archeops might look possible, but they both carry Crunch. Avoid Archeops, but Carracosta is a possible matchup. Burn it with Will-O-Wisp, and then finish it off with Grass Knot. Vanilluxe can use Hail and Blizzard. Stay away. Klinklang shouldn't be too dangerous, but the combination of Flash Cannon and Metal Sound hits harder than you want it to. Leave it to another Pokémon, if possible.
Ghetsis (N's Castle): If you have a higher leveled Cofagrigus than Ghetsis's, you could try battling it. It's not recommended, though, as Cofagrigus has way better matchups against Ghetsis. Bouffalant can hardly even hit Cofagrigus. Burn it with Will-O-Wisp, then switch out, because you can hardly hit Bouffalant too. Seismitoad is possible, thanks to Grass Knot. Just watch out for Rain Dance boosted Muddy Water, and you'll be fine. Bisharp knows Night Slash, big nope. Eelektross's moveset is mostly physical, so you could try crippling it with Will-O-Wisp. However, be on your guard for Flamethrower and Crunch, and don't be afraid to heal. Hydreigon doesn't know a Dark-type move for some unholy reason. Cofagrigus can use Will-O-Wisp or Toxic to wear it down, but honestly, you have nothing to hit Hydreigon with. Try to avoid.
Post-Game: Keep going like before. Keep Cofagrigus away from Dark-types and powerful special hitters, and you'll be fine.
The lowest leveled Yamask you can find in the wild come at level 19, at which they know Disable, Haze, Night Shade and Hex. Night Shade and Hex are useful at this point, Disable is nearly useless, and Haze is really situational, and honestly not worth it. If you catch Yamask at level 21 instead, it will know Will-O-Wisp instead of Disable. Will-O-Wisp is the best status move the Yamask line gets, and is an absolute must. At level 25, Ominous Wind comes. Ominous Wind has decent but not great power, and low PP. Hex is usually a better choice, unless you want to boost all your stats desperately. Curse comes at level 29. A dangerous move, but Cofagrigus is able to use it due to its incredible mixed bulk. Both Power Split and Guard Split can be learned at level 33, but they're near useless, and you'd hardly ever use them, if at all. Evolve Yamask as soon as possible, and Cofagrigus will learn Scary Face at level 34, which isn't a good move for it. Even after a Scary Face, most Pokémon will still be outspeeding. And you're usually not looking for a Cofagrigus sweep. Shadow Ball comes at level 39, and is next to Will-O-Wisp an absolute must in Cofagrigus's moveset. If you hadn't evolved Yamask yet, it learns Shadow Ball, at level 37. But it's still better to just go and evolve it, and wait for two more levels to get Shadow Ball. Grudge comes at level 45, and it will only have any effect when your Cofagrigus faints, exactly that what you want to avoid happening. Bad move. Mean Look is learned at level 51...nope, not good either. Then, Destiny Bond comes at level 57. You just don't want Cofagrigus to faint, though, so it's not worth a moveslot.
Sadly, the best of the Yamask line's useful TMs only come in the post-game. When you first catch Yamask, you can teach it Grass Knot, which you could have picked up in Pinwheel Forest. On Route 17, Payback can be found. The Yamask line's Attack isn't high, but it can be used to provide coverage. It's not truly great, though. Will-O-Wisp can be found in Celestial Tower, but honestly, Yamask/Cofagrigus should know it already. Then there's Shadow Ball in Relic Castle...which Cofagrigus should already know as well. Toxic comes on Route 17. Cofagrigus has the bulk to use the move, but usually Will-O-Wisp is a better solution. Hidden Power can be received from Professor Juniper when you have seen 115 of the Pokémon in Unova's regional Pokédex. Pretty much any type will be welcomed, as long as it's not Grass (covered by Grass Knot) or Ghost (covered by Shadow Ball and pretty much all other Ghost-type attacks). Preferred is Hidden Power Fighting, as it can be used to hit Normal-types hard. In the post-game, quite some useful TMs for Cofagrigus become available. It can broaden its offensive arsenal with Energy Ball and Psychic. It can become a pure mixed tank thanks to Calm Mind. And then there's Trick Room, which can be good for Cofagrigus for obvious reasons.
Recommended moveset: Will-O-Wisp, Shadow Ball, and any two between Curse / Toxic / Grass Knot / Hidden Power
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Yamask Line Ratings
What Nature do I want? A Defense booster is the best, but a boost in Special Attack or Special Defense is okay too. It's also preferred to have a Nature that lowers Speed, but an Attack drop works as well. Relaxed is usually the best, but Bold, Modest, Quiet, Calm and Sassy work very good as well.
At what point in the game should I be evolved? As soon as possible. Yamask's stats really aren't anything special, but Cofagrigus's are great.
How good is the Yamask line in a Nuzlocke? It's awesome. It has the mixed bulk to shrug off hits from some of the biggest threats in Unova, and can bulk even critical super effective attacks at times. Plus, it has a great type to be a physical wall, being immune to both Normal- and Fighting-type attacks. It also has a decent Special Attack, of which it can shoot Shadow Balls with considerable power.
Weaknesses: Ghost, Dark
Resistances: Poison, Bug
Immunities: Normal, Fighting
Neutralities: Fire, Grass, Water, Ice, Psychic, Steel, Dragon, Rock, Ground, Flying, Electric