I see 2 possible boosts:
1) Because it is really annoying that the random troop has a Barrier, add a dispel effect before devouring like Black Beast: Wulfgarok didn't need a nerf like Kerberos (in fact, no other "Devourer" need it...).
2) Increase his damage when he doesn't devour... something like that:
Deal [Magic + 2] damage to an enemy, boosted by Wargare Allies. If the enemy dies, devour a random enemy. If the enemy survives, deal [Magic + 2] damage to a random enemy, boosted by Wargare Allies.[4x]