http://www.smogon.com/dp/pokemon/dusknoir
If Uxie, Slowbro, and Porygon2 can have a Trick Room set, then surely Dusknoir should have one of the few things it can do that Rotom / Spiritomb can't written up, instead of being Other Options.
[SET]
name: Trick Room
move 1: Trick Room
move 2: Will-O-Wisp
move 3: Pain Split
move 4: Shadow Sneak / Earthquake
item: Leftovers
nature: Relaxed
evs: 252 HP / 252 Def / 4 SpD
ivs: 0 Spe
[SET COMMENTS]
<p>Trick Room sets Dusknoir apart from its bulky Ghost brethren, as neither Spritomb nor Rotom-A have access to it. Dusknoir is an excellent user of Trick Room, being both slow and possessing high defenses, and is a reliable choice for a supporter on a Trick Room team. The strategy is straightforward enough - simply come in on something that poses little threat to Dusknoir (bulky Fighting-types and most defensive Pokemon), set up Trick Room, and then switch out to a slow sweeper to abuse the three remaining turns of Trick Room.</p>
<p>Will-O-Wisp is a good move in general to have on a Trick Room team, as it not only cripples threatening attackers, but also weakens defensive Pokemon with its residual damage so that your sweepers can break past them. Tyranitar is also an extremely common switch into Dusknoir, and Will-O-Wisp allows you to slash its attacking power and reduce the threat it poses to your team. Pain Split provides Dusknoir with some instant, albeit unreliable, recovery, and if you choose your Pain Split targets carefully, Dusknoir should be able to stick around for quite a while and set up Trick Room multiple times.</p>
<p>There are many options that can go in the last slot. Shadow Sneak's priority is still sometimes useful in Trick Room and is definitely preferable to Shadow Punch outside of it, but you may feel you want the slightly higher base power for use in Trick Room conditions. Earthquake is a solid attacking move, and in particular hits any Heatran coming in to steal a Flash Fire boost from your Will-O-Wisp. It is recommended that you move 28 EVs from Defense into Attack if you choose to use Earthquake, as this guarantees an OHKO on a 4 HP Heatran with Stealth Rock. If you feel that Scizor, Salamence, or Gyarados are giving your team trouble, then you can use the appropriate elemental punch in that slot to help deal with them. While Pain Split is generally helpful, it isn't vital to the set, and another attacking move can easily be used in its place instead. Just beware, however, that Dusknoir will not last very long without any form of recovery, and may not be able to set up Trick Room more than once.</p>
<p>With a Relaxed nature and a Speed IV of 0, Dusknoir sits at a very slow 85 Speed, meaning it will outspeed the vast majority of opponents in Trick Room and can get some quick recovery with Pain Split.</p>
If Uxie, Slowbro, and Porygon2 can have a Trick Room set, then surely Dusknoir should have one of the few things it can do that Rotom / Spiritomb can't written up, instead of being Other Options.

[SET]
name: Trick Room
move 1: Trick Room
move 2: Will-O-Wisp
move 3: Pain Split
move 4: Shadow Sneak / Earthquake
item: Leftovers
nature: Relaxed
evs: 252 HP / 252 Def / 4 SpD
ivs: 0 Spe
[SET COMMENTS]
<p>Trick Room sets Dusknoir apart from its bulky Ghost brethren, as neither Spritomb nor Rotom-A have access to it. Dusknoir is an excellent user of Trick Room, being both slow and possessing high defenses, and is a reliable choice for a supporter on a Trick Room team. The strategy is straightforward enough - simply come in on something that poses little threat to Dusknoir (bulky Fighting-types and most defensive Pokemon), set up Trick Room, and then switch out to a slow sweeper to abuse the three remaining turns of Trick Room.</p>
<p>Will-O-Wisp is a good move in general to have on a Trick Room team, as it not only cripples threatening attackers, but also weakens defensive Pokemon with its residual damage so that your sweepers can break past them. Tyranitar is also an extremely common switch into Dusknoir, and Will-O-Wisp allows you to slash its attacking power and reduce the threat it poses to your team. Pain Split provides Dusknoir with some instant, albeit unreliable, recovery, and if you choose your Pain Split targets carefully, Dusknoir should be able to stick around for quite a while and set up Trick Room multiple times.</p>
<p>There are many options that can go in the last slot. Shadow Sneak's priority is still sometimes useful in Trick Room and is definitely preferable to Shadow Punch outside of it, but you may feel you want the slightly higher base power for use in Trick Room conditions. Earthquake is a solid attacking move, and in particular hits any Heatran coming in to steal a Flash Fire boost from your Will-O-Wisp. It is recommended that you move 28 EVs from Defense into Attack if you choose to use Earthquake, as this guarantees an OHKO on a 4 HP Heatran with Stealth Rock. If you feel that Scizor, Salamence, or Gyarados are giving your team trouble, then you can use the appropriate elemental punch in that slot to help deal with them. While Pain Split is generally helpful, it isn't vital to the set, and another attacking move can easily be used in its place instead. Just beware, however, that Dusknoir will not last very long without any form of recovery, and may not be able to set up Trick Room more than once.</p>
<p>With a Relaxed nature and a Speed IV of 0, Dusknoir sits at a very slow 85 Speed, meaning it will outspeed the vast majority of opponents in Trick Room and can get some quick recovery with Pain Split.</p>