In this quest, you encounter Karlach, a devil whose soul was forcibly sworn to Zariel who escaped from Avernus by riding the nautiloid. At some point, before the crash, she was infected.
Whoot? Have I missed something here?
Or brought the wrong compagnions and couldn't figure that out?
To me it looked like she is just broke out of Avernus Jail - the Mindflayers could not get their hands on her since she just hid on the Nautiloid and she is not a Devil either, but a Tiefling.
Devils, especially more highranking one's with Human seize and shape, they all phased back to the Hells when we made the jump.
Proof that their "devil heritage" is so high and dense, they cannot reach the Material realm -> without a proper dark summoning ritual of sorts.
Karlach could, which could serve as proof that she is a Tiefling and not a "real Devil".
Her pursuers just call her a Devil to lure US the players into aiding them.
And all those superstitious fools in the world of Fae'run call Tieflings Devil cause they just dont know any better.
Zariel that dumb "biatchu" is just mad one of her Slaves could run off.
... is as I understood it.
