I haven't seen any discussion about trevenant recently, and I think it's going to be pretty pivotal in the metagame.
Here's the set I've been using - it's very rough, but I've only just made it.
Trevenant @ Sitrus Berry
Ability: Harvest
EVs: 252 HP/252 Atk/4 Spe
Nature: Adamant
- Substitute
- Leech Seed
- Will-o-Wisp
- Horn Leech
Explanation: Trevenant fit perfectly on my stall team as a pseudo-sweeper and a spinblocker. He is perhaps the best spinblocker currently in UU, and with the loss of common Defog users like Zapdos/Latias and the rise of new spinning toys to play with (Forry/Starmie/Tenta/Donphan) as well as existing spinner like M-Blast and HTop, Trevenant is all the more effective as a spinblocker. Forry, Hitmontop, and Donphan can't touch him bar a predicted toxic. As for the many of the remaining spinners, even if they run an appropriate move to deal super effective damage the result is paltry at best.
0 Atk Forretress Gyro Ball (31 BP) vs. 252 HP / 0 Def Trevenant: 39-46 (10.4 - 12.2%) -- possible 9HKO
252+ Atk Hitmontop Sucker Punch vs. 252 HP / 0 Def Trevenant: 194-230 (51.8 - 61.4%) -- guaranteed 2HKO
252+ Atk Donphan Ice Shard vs. 252 HP / 0 Def Trevenant: 114-136 (30.4 - 36.3%) -- 54.1% chance to 3HKO
0 SpA Tentacruel Sludge Bomb vs. 252 HP / 0 SpD Trevenant: 96-114 (25.6 - 30.4%) -- guaranteed 4HKO
0 Atk Tentacruel Knock Off (97.5 BP) vs. 252 HP / 0 Def Trevenant: 132-156 (35.2 - 41.7%) -- guaranteed 3HKO
252 SpA Life Orb Starmie Psyshock vs. 252 HP / 0 Def Trevenant: 177-211 (47.3 - 56.4%) -- 81.3% chance to 2HKO
The two spinners that can deal with Trevenant (at least this set) are Starmie, with Ice Beam, and Mega-Blastoise, with Dark Pulse. However, if you correctly predict and switch in on a Scald/rapid spin from Starmie, the threat of a Horn Leech will be more than enough to send it packing (or, just straight up kill it). The same could be applied to Mega-Blastoise if you ran more SpD, but I personally prefer the Attack.
The combination of Substitute and Sitrus Berry Harvest allows Trevenant to stall out even the most offensive counters if he is given even just a turn of momentum. You can even potentially stall out a Victini if it predicts correctly and locks itself into V-Create, merely by spamming Substitute until its stats are lowered to the point where you can outspeed, leech seed behind a sub, and theoretically continue to sub up provided you get reasonable harvest rolls. (This is obviously an extreme and pretty situational example, but it's just meant to show the power of Trevenant.
Leech Seed offers consistent damage and healing, and makes Trevenant even more annoying to kill than he already is.
I chose Will-o-Wisp because my team has toxic spikes support already. Will-o-Wisp allows me to cripple levitating, steel, or flying type threats, both defensive and offensive. Bulky defensive threats like Aggron, Rhyperior, or Jirachi become less capable of breaking your sub, allowing for more stalls. Moreover, Physical T-T, Hawlucha, Crobat, and other offensive threats won't expect the will-o-wisp and will lose much of their threat to your team as well.
Horn Leech combined with full investment in a attack is a two-edged sword. Not only does it mean your attacks do ridiculous damage for a utility/defensive based set, it also increases the amount of healing you get to an unfathomable amount. As an example-
252+ Atk Trevenant Horn Leech vs. 252 HP / 0 Def Florges: 165-195 (45.8 - 54.1%) -- 3.9% chance to 2HKO after Leftovers recovery
252+ Atk Trevenant Horn Leech vs. 252 HP / 252+ Def Krookodile: 200-236 (50.7 - 59.8%) -- 84.4% chance to 2HKO after Leftovers recovery
Trevenant works best (somewhat obviously, as he is a spinblocker) with extensive hazard support, which allows him to accumulate more damage and gain momentum more easily. In general, trevenant should be switched into things that can't do all too much to him so that he can start setting up and wreaking havoc on the enemy team (ex. Florges, Mega-Aggron, Forry, Smeargle).
Some things to watch out for: Umbreon can easily OHKO Trevenant with this spread with not much previous damage. Obliterate with caution.