This is for completing the prologue. It's pretty straightforward and takes about an hour. There's nothing connected to the other achievements in this chapter, although your choices at the end will decide who joins your party when.
Amriri the Barbarian will join your party no matter what. She's a strong damage dealer, but needs healing often.
If you choose a Good option, Linzi the bard will join your party. She has minor damage, some healing and some buffs, plus she's the only one who can open locks and remove traps.
If you pick Evil instead, Jaethal the inquisitor will join your party. She is weird, but can be taught to heal and do moderate damage or buffs.
If you pick Lawful, Valerie, the tower shield fighter, will join your party. She is the best tank.
If you choose Chaotic, Harrim the cleric will join your join your party. He is a good healer and buffer, and can do some melee damage.
Your best choice will depend on matching your character's strengths, but I strongly recommend choosing good and getting Linzi.