Ah yes, the typical Halloween Ghost-type for Pokemon of the Week. And its the Pokemon you can't trade over GTS without nicknaming it because it has a cuss word in its English name... Cofagrigus.
I really like this Pokemon. It has a crazy-high base 145 Defense stat, an above average base 105 Sp. Defense stat, and a decent base 95 Sp. Attack. On the downside, its defensive abilities are somewhat crippled by a rather low base 58 HP stat, and it sits at a sluggish base 30 Speed. However, that low Speed can work to great advantage to Cofagrigus if you use Trick Room, which it can set up itself. It also gets Nasty Plot and Calm Mind to enhance its Sp. Attack, increasing its sweeping possibilities. You can even run both Trick Room and a boosting move on the same set, because Cofagrigus gets perfect coverage with Shadow Ball and Hidden Power Fighting.
Even with low HP, those massive Defenses can let Cofagrigus take many hits before being taken down. Like many Ghost types, it also gets Will-O-Wisp, which cripples any physical attacker that isn't a Fire type or lacks Guts. However, it lacks any form of recovery outside of Rest. It can't even learn Pain Split, which is a great disappointment. A reliable cleric is a good thing to have if you run a defensive Cofagrigus.
The most unique thing about Cofagrigus, however is its ability: Mummy. The only Pokemon who get this ability are Cofagrigus and its pre-evolution, Yamask. If a Pokemon makes physical contact with Cofagrigus, their Ability becomes Mummy as well. The abilities are reset to normal if there is a switch of Pokemon, however. As Cofagrigus will usually be brought in against physical attackers (and nearly all physical attacks make physical contact), this ability is of great use. Getting rid of Darmanitan's Sheer Force, Conkledurr's Guts, or the infamous Iron Head Jirachi's Serene Grace will severely hinder the opponent and likely force many switches. This makes for great synergy with entry hazards. Mummy is also fantastic in Doubles and Triples, as you can have a Slaking, Regigigas, or Archeops attack Cofagrigus to get rid of their crippling abilities. On the other hand, if you come across any of those Pokemon in singles, you probably will want to switch Cofagrigus out.
Confagrigus also gets two thinly spread moves: Power Split and Guard Split. These moves work like Pain Split, but with the Pokemon's offensive and defensive stats, respectively. I'd say these moves are best utilized in Double and Triple Battles; sharing power and/or guard with an ally can be a very effective strategy (I actually have an entire team based off of those moves).
No matter how bulky a Pokemon is, it will hate Toxic if it lacks Rest. This holds true for Cofagrigus as well. It also detests Taunt unless it runs a Choice set, which you probably will never see because of a limited offensive movepool. As for actually taking it out, Sp. Attacks are recommended, as its Sp. Defense is much lower than its physical. LO Gengar Shadow Ball is a good example. If you come across the Trick Room set and lack a Trick Room user of your own, it becomes tricky, especially if Cofagrigus has gotten off a Sp. Attack boost or two. It is immune to four priority moves: Quick Attack, Mach Punch, Extremespeed, and Vacuum Wave. Sucker Punch and Shadow Sneak can work, but even then Cofagrigus can still take a number of hits before falling. Stalling out Trick Room is the best idea in that case.
I know Cofagrigus is considered RU, but I think it can compete with the bad boys of OU just as well because its positives are greater than its negatives. Underestimate this coffin at your own peril.
Yamask is one of my favorite Little Cup Pokemon. It's just so adorable. It can run just about anything Cofagrigus can run, and does it quite well thanks to its impressive defenses and good Sp. Attack. I recommend using it on a Trick Room team, as it is slow.