The Evilest thing you can do in BG3...

[Ael]

In my honest-man play-through (not reloading no matter what) the biggest regret I had up until now and nearly made me break this rule was when I broke up with Karlach to pursue Shadowheart.
Seriously Larian, your writers and VA deserve the highest of praises.
I've never felt this much regret and sadness on a decision in a video-game, the feeling she passed was so real.

Thank you for delivering us such incredible experience

*TLDR - Break with Karlach, if that doesn't sadden you nothing in life will.

[アンジェル]

I read that Shadowheart has the most rewarding romance story among all companions - that is, if you manage to win her heart.

And Karlach... yeah... I still like her ending more. Or rather, it moved me so much ...