Cofagrigus is a great offensive Trick Room sweeper. Thanks to Mummy, it can also act as an emergency check to dangerous threats such as Mega Medicham and Mega Pinsir, and Cofagrigus's presence alone deters High Jump Kick spam. Due to the lack of Dark- and Normal-types in OU, Cofagrigus can come in, start boosting with Nasty Plot, and attempt to sweep. Once boosted, Never-Ending Nightmare allows Cofagrigus to muscle past a strong target once, and the threat of the Z-Move puts pressure on the opponent to absorb it before it is used. However, Cofagrigus is rather weak before it sets up, and Tyranitar and Bisharp can tank its hits and Pursuit trap it. Also, Cofagrigus has no recovery option, so it will not be able to sweep if it is used as a defensive check prior in the match.
name: Trick Room Sweeper
move 1: Shadow Ball
move 2: Nasty Plot
move 3: Will-O-Wisp / Hidden Power Fighting
move 4: Trick Room
item: Ghostium Z
evs: 248 HP / 252 SpA / 8 SpD
Shadow Ball is Cofagrigus's only attack, but the lack of prominent Dark-types and Normal-types make it a very spammable move. Nasty Plot boosts Cofragrigus's Special Attack to high levels, making it very hard to switch into and easy to initiate a sweep. Will-O-Wisp cripples attackers that Cofagrigus can check such as Mega Medicham, Mawile, Garchomp, Zygarde, and Gyarados. Trick Room transforms Cofagrigus into the fastest Pokemon on the field, allowing it to clean up a team without enough bulk to take its hits. Hidden Power Fighting is an option over Will-O-Wisp as a way to hit Dark-types, as well as giving Cofagrigus perfect neutral coverage.
Maximum HP investment makes Cofagrigus as bulky as possible for the ease of switching into attackers and setting up. The rest is placed into Special Attack so it can hit as hard as possible. A Quiet nature paired with 0 IVs in Speed ensures that it is the slowest Pokemon on the field. Ghostium Z gives Cofagrigus the ability to muscle past frail Dark-types at +2 such as Greninja, Ash-Greninja, and Weavile. It also assists with breaking through bulky Pokemon such as Assault Vest Tangrowth and Magearna. If you already have a Z-Move user, Leftovers is a fine option for passive recovery, although Ghostium Z is usually superior.
It's best to preserve Cofagrigus until late-game to sweep, as it appreciates the opposition being weakened. However, if Cofagrigus has teammates that can take advantage of Trick Room, Cofagrigus can set it up and start boosting earlier in the match. If necessary, Cofagrigus can be used as an emergency check to powerful threats such as Mega Medicham and Mega Heracross thanks to Mummy nullifying their abilities. When Cofagrigus is already in battle but is unable to begin a sweep, simply use Will-O-Wisp to cripple and further wear down the opposing team.
Cofagrigus fits best on offensive teams that appreciate its immense standalone sweeping potential and need it as an emergency check to specific threats such as Mega Medicham and Mega Mawhile. Thus, Cofagrigus greatly appreciates being paired with Spikes setters such as Greninja, Ash-Greninja, and Ferrothorn. Cofagrigus also values a Bisharp and Tyranitar check, as Bisharp is one of the few Dark-types that can KO it after prior damage regardless of whether or not Trick Room is up due to Sucker Punch, and Tyranitar is very bulky and takes negligible damage from +2 Never-Ending Nightmare. Keldeo, Terrakion, Magearna, and Mega Medicham are good options to take these Dark-types out. It's worth noting, however, that Mega Medicham is unable to safely switch into Dark-types. Dugtrio is a decent partner, as it traps and eliminates Tyranitar, Bisharp, and Chansey. Partners such as Mega Mawile, Magearna, Azumarill, and Crawdaunt appreciate Trick Room, so they worth considering, as Cofagrigus sets up Trick Room rather reliably.
A defensive set is plausible, as Cofagrigus has tools to pull it off. If one wishes to use such a set, an EV spread of 248 HP / 252 Def / 8 SpD with a Bold Nature is optimal with a moveset of Hex, Pain Split , Will-O-Wisp, and Toxic Spikes. Haze is also an option on this set to check boosting attackers.
Checks and Counters
**Normal-types**: Aside from burning them with Will-O-Wisp, Cofagrigus lacks a way to get through Normal-types such as Chansey and Smeargle. Both can cripple it with Toxic and Spore, respectively.
**Dark-types**: Although Cofagrigus can break through most of them with +2 Never-Ending Nightmare, Bisharp, Weavile, and Tyranitar are big threats. Bisharp has access to powerful priority in Sucker Punch to stop Cofagrigus even in Trick Room, and Tyranitar is bulky enough to take +2 Never-Ending Nightmare. They must, however, watch out for Will-O-Wisp.
**Taunt**: Cofagrigus is completely shut down by Taunt. Without access to Nasty Plot, Trick Room, and Will-O-Wisp, it's rendered useless bar relatively weak Shadow Balls.
**Unaware Users**: Although Cofagrigus can muscle past physically defensive Unaware Clefable, more specially defensive variants can survive Shadow Ball followed by Never-Ending Nightmare. However, Shadow Ball boasts a chance to lower the foe's Special Defense, so specially defensive Clefable could lose one-on-one.