Short: Right now I’m thinking about our next session and how to handle a PC who has attacked us the second time with an AoE spell. To defend him: he’s denying that he has done it intentionally and even saying that he doesn’t know anything about it.
First: I don’t want/need the best solution, just some ideas – like "how would you react?".
But I will start by describing the whole situation:
We are a group of 5 PC – there are two double-pre-mates and I’m the lonely fifth one. Two sessions ago the fighter in question attacked two of us the first time while we explored a mine occupied by wererats and some following smaller ones. The first small rat attacked him, as he’s always the one going first. Our archer, the rat and I were attacked by black tentacles getting out of the face of our knight and/or the rat – as the rat bite him in his face. The rat was slashed in three parts and the archer and I were lucky enough to dodge. After we cleared the room we surrounded the fighter and asked him what it was, if he has any explanation or why he did it to us. He has only answered that he doesn’t know what it was and he hasn’t done it. We told him that we will watch him and aren’t really trusting him anymore.
Some days (in-game) later we got to an old cathedral occupied by orcs and ogres. We attacked the orcs with the help of the wererats, as the cathedral is the real home of them. While fighting the first ogre (the fighter his accompanying cleric and I) these black tentacles popped up again, this time clearly out of his mouth, and attacked the ogre, an orc, the cleric and me. All were damaged (necrotic) and also prevented reactions for one round as we had to get rid of these tentacles first.
During the fight we haven’t talked about it as there were a lot more orcs than we thought first. The end of our last session was that we have "finished" (we haven’t checked the cathedral – only surrounding) the fight. We’ve lost some wererats and we all are heavily damaged, nearly all spell-slots are used and we have this big-black-mysterium in our group.
As it was the second time, this time we were damaged and we’ve clearly seen that the origin of these tentacles is the fighter we have to react somehow.
My current idea is to call the archer, as he still doesn’t trust the fighter and was betrayed in his past, so he will very likely support me. After this I would like to approach the fighter and "command" him to drop his weapon and get on his knees. I have placed a force-ballista (artificer) during the battle and can use my crossbow. If he doesn’t follow we get to the first problem – if I really attack him he will get very likely unconscious as he has 4HP remaining. Should I? I’m a lawful-neutral/good dwarven artificer, trying to get part of "Hammers of Moradin" and waiting for a sign of Moradin himself that I’m worthy to join the hammers.
So from my perspective it would be possible for my PC to attack a presumably lying, betraying, team-attacking, necromancy casting human fighter.
But in success-case, he’s now on his knees, I would at first arrest/tie him, ask him again in hope that he will talk more about these tentacles and otherwise transport him to the next town to present him to the local guards or similar and get him judged.
Now the question: do you have other ideas? How would you react? Everything is possible/valid response – as I’m pretty new to DnD (it’s my first campaign) and I want to get some ideas about what else could be possible.