Baldur's Gate 3's Patch 7 adds additional evil endings for each of the origin (playable) characters.
All of the evil endings are comically so, but taking control of the Netherbrain and using it to dominate an entire population of people was never going to be all sunshine and rainbows, was it? Karlach's evil ending, in particular, has been attracting attention for how brutal it is.
We first see the Tiefling looking down over a burning Baldur's Gate from atop the Netherbrain. Then, she descends to a gallows to address a group of the city's citizens, a scene that's common to all of the new endings. If you choose the dialogue option "Fuck Baldur's Gate. Fuck Avernus. They're mine now," then the brutality commences.
Karlach is originally from Baldur's Gate. She worked as Enver Gortash's bodyguard to support her parents until he betrayed her and sold her to Zariel as a test subject for the archdevil's new infernal machine.
Karlach points out her perceived betrayal by the people of Baldur's Gate. This is for two reasons, the people of the Sword Coast are generally prejudiced against Tieflings, and they allowed Enver Gortash, her betrayer, to ascend to the rank of...