Baldur's Gate 3: Obliteration

[Part of the Cursed story line]

The party group up with Gortash to head to the elder brain's lair and try re-establish control of it with the netherstones. This ultimately fails as the brain has evolved into a netherbrain which power word kills Gortash. The D20 warps the party out of there as the brain begins to float up to the city, turning the many infected people into mindflayers while also summoning more nautiloids from realm space to take over the world. To combat this the D20 manifests itself as a mindflayer too and suggests one of the heroes also goes mindflayery.

Karlach volunteers, turning into a cute squidface. They then manage to meet up with their allies: Councillor Florrick who offers three soldiers, three vampire hunters, Volo and Sarevok (those last two just give passive buffs) before heading into a massive battle against pretty much everything they've fought up to this point but it is Varanna who is the MVP with her bow and a lot of exploding arrows (which she can shoot multiple times per turn apparently).

In the end it is Gale who is the hero though, climbing up onto the brain as it hovers far above Baldur's Gate and self detonates his bomb - obliterating it from existence. This causes fatal headaches to the enemy flayers and giving the citizens the upper hand against the invaders. Karlach and the D20 then go into hiding, Varanna returns to working for the guild, and Catharina (who went against Bhaal's wishes of dominating the brain) quickly degrades into a killer flesh puppet for the god of murder.