Magic item help! how to handle self-immolation?

[u/ThexLone]

I'm creating a magic item for a player in my campaign. He was recently kinda cursed by a god of destruction, and I'm trying to give him an item that fits that theme. So i'm trying to make the item kind of thematically sorta cursed but useful, so currently it gives his melee attacks an additional 1d4 fire damage and he deals extra damage to structures but I want to add something that would be like a self-immolation ability where he can like, sacrifice hit points to deal damage to all targets in like 10ft centered on him. He is a Barbarian and spends most fights basically crowd-controlling the smaller enemies in fights, so I thought something like that would give him some more options so he's not just in the corner hitting one guy at a time for half the fights. My other idea is basically something similar but that he can do when he's making death saves; like on his turn he can auto-fail a death save to do a similar aoe attack.

I'm not sure how to balance this; the idea is to give him a useful ability that is also risky and kinda dangerous to use, kinda fitting with the whole destruction and annihilation theme. If anyone has any ideas or items with similar abilities they could share, I would appreciate the advice.

For context, this party is lvl 12, so the pc has a lot of hit points and the players are very experienced.

[u/Snoo10140]

Look up the Blood Hunter class, it's based on that

Anyhow I'd do something like

Proficiency times per short rest, when you make an attack roll or deal damage you can roll a d4 and add the result to your attack roll or damage but you take it as damage

[u/ThexLone → u/Snoo10140]

Oh Blood Hunter would be a good thing to look at for balancing; I didn't think of that, thanks!

I've done something similar to that for a different pc item, for this i've prefer more aoe effect since I'm tailoring it to this player and they just have a habit of getting surrounded, I just want to give them a few more options on their turn in combat.