Diablo II: Resurrected

This pops immediately upon defeating Andariel, the boss of Act 1.

Andariel is on the 4th floor of the Cathedral. By far her most powerful attacks are poison-based, which with low or no resistances can take unprepared characters from full health to dying very quickly. You can mitigate this some by wearing some poison resistance gear, or having some charms in your inventory, if you've found any. Otherwise, you can use Antidote potions for an immediate cleanse, healing potions to counteract the health loss, rejuv potions for immediate restoration if you're getting low, etc. Antidote potions can be bought from Akara in the Rogue Encampment, and if consumed before you start the fight provide a period of improved poison resistance. Multiple potions increases the duration, not the size of the effect. Buying and consuming one or two before the fight will help. Another one or two in the belt should be more than enough.

A common tactic is to clear the antechamber, trigger Andariel and then lure her into the antechamber rather than fighting her where she is. While it's smaller which makes it harder to kite and dodge, you avoid having the extra enemies in the main area as adds.