Update 2: After further review, Larian has decided that this is in fact a bug, and plans to remove it "ASAP."
Update: Divinity: Original Sin 2 producer David Walgrave tells me that Larian doesn't plan to patch out this technique because it's "part of the system."
"It appears that it is not a never-ending loop and you're not doing anything else while you're doing it," wrote Walgrave. "It's not like you're damaging anything for free without spending action points. If I understand this trick, it's still within the purposeful synergies of our skills and usually these are accounted for."
That's true: you can't do anything while the loop runs its course. Though, based on the video, which shows a troll losing 6048 HP like it's nothing, it's obviously quite a loop, technically 'never-ending' or not, and I've asked for further clarification. For now, rather than 'fix' the combo, Walgrave says they "applaud" it. Examples of techniques that would be patched out include anything that "breaks the fun" or is actually a bug. "If this would tick in realtime, it would be a bug," wrote Walgrave. "And if this does not cost AP/skills and/or is not blocked by cooldown/consuming a scroll/... then we look into it."
Otherwise, "If you glitch the system, congratulations," concludes Walgrave. So there you have it, feel free to combo trolls to hell all you want. The original story and explanation of the trick follows.
Original story: One of the magical things about Divinity: Original Sin and now its sequel, Original Sin 2, is how complex and liberal their combat systems are. Whether by design or by accident, the glut of spell and physics interactions sometimes lets you get away with ridiculously OP moves. In the first game, for instance, players leveled up telekinesis, made absurdly heavy objects by stacking containers, and then dropped them on enemies for unreasonable amounts of damage. Original Sin 2 now has its own trick, courtesy of Ashandis on Reddit (and others), except this one appears to deal unlimited damage to a single enemy.
You can see the trick in action in the video above. How it works relies on several complimentary spells, as well as a quirk particular to undead characters: they take damage from healing spells.
So here's the trick. You have one party member cast Soul Mate on an undead party member. This spell will cause the undead character to receive half of any healing the caster receives, which will actually damage them. But that's OK, because you've already had the undead character cast Living on the Edge, which will prevent them from dying for two turns, no matter how much damage they take.
The caster of Soul Mate must have a passive ability called Life Leech, which will heal them every time they deal damage. And that's how you get the loop. Say I'm the original caster of Soul Mate: I now cast a healing spell on myself, the undead character takes half that healing as damage (Soul Mate), I receive more healing because I damaged the undead character (Life Leech), they receive more damage because I was healed again, and on and on it goes.
Of course, that doesn't help you, as you're just dealing unending damage to one of your party members. So there's another ingredient: the undead party member uses Shackles of Pain to deal all the damage they take to an enemy.
According to Ashandis, the loop doesn't prevent you from taking turns, so it can be ended by letting Soul Mate wear off. I'm not at a place where I can try it myself, but the abilities are all real, and the methodology makes sense.
There's no word on whether Larian intends to patch out the loop, or if it even considers it an exploit—there are lots of ways to 'break' Divinity's systems, and some are intentional, as Fraser discusses in our glowing review. This one may be a bit too OP to keep around, but you never know. I've reached out to Larian for comment.