A Lost Soul - NWN Side Quest (Chapter 1)
Beggar's Nest: A Lost Soul is a side quest that begins by speaking with Bertrand inside of the Temple of Helm in the southern portion of the Beggar's Nest. You can also begin this quest by picking up Marcus' belongings on his body in the northern portion of the Beggar's Nest District.
For this quest Bertrand asks you to find out what happen to his brother Marcus. You can find Marcus' corpse in the northern portion of the Beggar's Nest area, his corpse is just outside of the gate that will lead you into the Great Graveyard. If you hold down TAB to highlight all of the objects you'll see that his corpse has a unique name and stands out from the other objects nearby.
If you still need help locating him use my screenshot below, I have the map open in it. When you turn in this quest you have the option of only giving Marcus' book to Bertrand if you want. However, you'll get EXP for each item you turn in so I recommend that you give him both the book and the staff. The EXP you earn is far more valuable than some gold.