I'm seeing some strange bugs in multiplayer, namely seeing Seth appear underground as early as the beginning of the game, though he doesn't do contact damage, move, or take damage, only sometimes making fire trails and causing intense visual bugs like making terrain go dark, or causing a lasting lens flare effect. I suspect it's just a normal enemy that's bugged into being seth, since the host didn't see him and was able to kill him, dropping a burning coal.
Other similar examples include seeing darknuts, moonwalkers, and sheep appearing early game. The death message and behavior of the moonwalker (it teleports around the screen like it's lagging, but cannot pass through walls and resembles the wisp charge) in particular claims that it's a rebellious wisp, but it does 130 damage on contact, which i assume comes from the moonwalker. The host also does not see the moonwalker, nor the hadouken-firing sheep.
Update: Unfortunately, I'm going to have to unrecommend this mod to others, given how the bugs have gone from annoying to dangerous. Seth alone appearing randomly underground with great frequency and causing all manner of visual bugs is annoying enough, but entering the Underworld Domain turns the visual bugs into seizure-inducing flashing, some of the strongest i've ever seen, enough that they gave me a headache despite no history of epilepsy. Given that the mod has seemingly gone quiet (and therefore it seems unlikely to be fixed for quite some time), I'm going to have to recommend people stay away from it for their own health.