Like said by Trapinchinator, the stall set is better in most cases for this tree. But Drain Punch is situational at best for an offensive Trevenant. Since it's in UU, take a look at what Drain Punch can hit supereffectivley.
Absol, Aggron, Empoleon, Hydregeion, Lucario, Umbreon, Kyurem, Mega Houndoom.
252+ Atk Trevenant Drain Punch vs. 252 HP / 4 Def Umbreon: 146-174 (37 - 44.1%) -- 99.9% chance to 3HKO after Leftovers recovery
Standard specially defensive Umbreon. Not that good, since it can just easily heal the damage off because it's Umbreon.
252+ Atk Trevenant Drain Punch vs. 0 HP / 0 Def Hydreigon: 176-208 (54.1 - 64%) -- guaranteed 2HKO
Scarf Hydregeion. Dark Pulse does around half and an LO variant can deal 70-80% if you're running Assault Vest on it. Without AV, the obviously faster Hydregion deals 80-94% scarfed and an OHKO with LO.
252+ Atk Trevenant Drain Punch vs. 252 HP / 136+ Def Aggron: 160-192 (46.5 - 55.8%) -- 76.2% chance to 2HKO
Even less when it's Mega evolved. But Aggron is the reason to run Drain Punch if it bothers you that much if you want to that badly if you're a Sitrus Berry set.
Drain Punch Trev is also solid against Lucario, but they can't exactly touch you anyways, so it's better to use that moveslot on something else. It also gets EQ, so yeah.
252+ Atk Trevenant Drain Punch vs. 252 HP / 0 Def Empoleon: 180-212 (48.3 - 56.9%) -- 36.3% chance to 2HKO after Leftovers recovery
Eh, Earthquake just might be better, as it's an actual 2HKO, but it's good if you're running AV against said Empoleon. Scald burns are a thing though, and without that AV, Ice Beam is a solid 2HKO.
As for Absol, Kyurem, and Houndoom, who would stay in on those with Trevenant.
I would say it's not worth it if you're just going for a standard Trevenant, as the STAB Horn Leach is usually better. But Drain Punch can be used if it covers up holes in your teams offense.
