Last generation, Dusknoir found itself struggling to stay in OU, and with the introduction of many powerful threats in BW, it has finally made the drop to UU. This new generation has replaced Dusknoir's old competition, the Rotom formes, with new Ghost-types, namely Cofagrigus, Golurk, and even Dusknoir's pre-evolution Dusclops.
However, Dusknoir is far from unusable this generation. It functions very capably in UU, with some of its spinblocking competition, namely Jellicent, restricted to OU. Dusknoir can also be used over its pre-evolution due to the fact that it can actually cause direct damage without relying on Night Shade or Seismic Toss, and it can also carry Leftovers, which is a major benefit overall. Dusknoir can be used over the likes of Cofagrigus or Golurk because of its higher overall defenses (although only its Special Defense is higher compared to Cofagrigus), and because it has Pressure, which is always a respectable ability. Dusknoir can still perform well despite the setbacks it faces this generation.
Dusknoir's standard set from last generation is back, albeit changed around a bit to suit the UU environment. Its main advantage over Dusclops is its ability to actually hit back after tanking damage, mostly to Pokemon immune to Will-O-Wisp, which can be used to shut down most of the physical attackers in the tier. Pain Split is for recovery, and is quite useful, due to Dusknoir's low base HP stat.
The last two moves are for defeating potential counters. Shadow Sneak can be used to pick off weakened foes. Return is better than Earthquake in this tier since it can hit Flying-types, and is usually recommended; Earthquake generally is only useful for defeating Flash Fire Pokemon that come in to absorb Will-O-Wisp.Team Options & Additional Comments >>>
Dusknoir can run a SubPunch or Choice Band set, but these are generally outclassed by Golurk in UU due to the latter's higher Attack and Iron Fist. A defensive Trick Room set is also largely outclassed by Dusclops, but Dusknoir could make an average Trick Room sweeper.
Dusknoir has many other status moves it could use to some success. A Torment set could work due to Dusknoir's high defenses, but the lack of resistances could be somewhat off-putting. A RestTalk set could work, especially since the Rotom formes are no longer Ghost-type, but you could run into a Pokemon that could KO you while you are sleeping before you can use Rest again. Calm Mind could work to fake out opponents that usually expect physical Dusknoir, but Spiritomb and Cofagrigus are better Calm Mind users due to their higher Special Attack. Counter could surprise Pokemon wishing to 2HKO Dusknoir, but it is risky, especially without Wish support.
Checks and Counters
Pursuit users can often seriously dent Dusknoir if they're not burned. Hard-hitting atackers immune to or even benefiting from Will-O-Wisp, such as Heracross or any Flash Fire user, can get a free switch in and KO Dusknoir.
Defensively, any Pokemon using Substitute can block Will-O-Wisp and Pain Split, although Dusknoir can break through the Substitutes. Spiritomb can wall Dusknoir and beat it with STAB Dark Pulse. Status moves will wreak havoc on Dusknoir without Rest or cleric support. Xatu in particular is dangerous, as it can reflect Will-O-Wisp back at Dusknoir.