Baldur's Gate 3 Players Have Somehow Killed Mizora And Withers

thegamer.com:

Withers and Mizora are basically immortal in Baldur's Gate 3. Withers is an old god so, yeah. Mizora is a devil holding Wyll hostage with a nasty contract. They're not exactly lowly goblins you find on the road, they're the closest the game has to 'essential NPCs'. But that hasn't stopped players from figuring out how to cut their time in the Sword Coast short.

"I thought [Withers] couldn't be killed," CAPHILL posted to the game's subreddit. "Used Ability Drain to reduce his INT to zero and he died. We decided not to load and see how it impacts the 'gathering' speech. After a long rest, his loot turned into a pouch. Guess he's not coming back."

It's okay. Just go back to camp and resurrect him. May cost a bit but I'm sure Withers will- oh.

A week later, DTraitor gave the method a try with another 'immortal' NPC - Mizora. You can technically kill her in Act 2 (read: send her back to Avernus) by activating the buttons beside the Mind Flayer pod she's trapped in. The problem is that this also kills Wyll (read: sends him to Avernus). Nevertheless, the pod at the end of Act 2 is your only opportunity to off Mizora as she's meant to be unkillable when she stops by to gloat in camp.

Drain Ability (INT) plus Absorb Intellect, however, worked a treat. DTraitor turned her into a pile of ash and Wyll wasn't dragged kicking and screaming into the bowels of the fiery inferno that is Avernus.

Of course, since she's a fiend, 'killing' her is only temporary since she'll eventually piece herself back together again. I doubt Larian considered people bypassing the whole 'unkillable' rule, so I wouldn't worry too much about a vengeful devil popping up while you long rest. Still, the lingering threat of a peeved Mizora will forever hang on