I like the idea it might need some sort of time limit applied otherwise the Human players could just fight the Infernal player forever, with no one winning or losing, although I guess the Humans would lose lives eventually, even if very slowly.
(to balance it off between the human player, give the infernal enough power to potentially hold back both players, but not too OP so that they can completely obliterate any one player if they choose).I would kind of suggest you want the opposite of this; the Infernal player could destroy 1 player, but struggle against both. The reasons for this are:
Easier balancing - Trying to balance something to be good against two but not against one is tricky. You could go for more small units so it's number over power, but there's only 11 trogs in the game and it's not too hard to get them all out as it is, so offering free trogs might prove redundant eventually.
Human Inter-play - You can add an extra element to the game play where the Human players know they need to team up for at least some of the game so that either of them have a chance of winning. When to turn on the other Human player is a fun decision point.
Give the Infernal player a chance to win - Further to the above point, if the Humans get the balance of teamwork vs relic hunting wrong, the Infernal player should be able to capitalise on this. If one Human player just watches the other get slaughtered, that's on them and they'll need to be prepare to fight the Infernal player alone (and probably die).
The challenge lies in making it balanced in such a way that the Infernal player still has a chance to win against both Human players teaming up throughout the game, and that a single Human player still has a chance to escape against the Infernal player.