The Hunt for Valygar Corthala - Baldur's Gate 2 Side Quest
The Hunt for Valygar Corthala side quest begins by speaking with Tolgerias inside of the Council of the Six building in the Government District of Athkatla. Aside from speaking to Delon in the Government District and accepting Investigate the Deaths in the Umar Hills, this side quest is the only way to get the World Map location for Umar Hills.
You'll find Valygar inside of Valygar's Cabin in the northern portion of Umar Hills. When you arrive he'll fill you in on his family's secret and the fact that his blood (or body) will give you access to the Planar Sphere dungeon in the Slums District of Athkatla. At this point you have 4 ways to complete this quest, one of which I do not recommend at all.
Valygar Quest Outcomes:
1. He joins your party and you go to the Planar Sphere to slay some lich
2. Tell him to go home to his home in the Docks District of Athkatla
3. Do not add him to your party and attack him. Pick up his body and use that to enter the Planar Sphere
4. Do not add him to your party and attack him. Pick up his body and deliver it to Tolgerias for 11k EXP and 500 Gold
The last outcome, to kill him and deliver his body to Tolgerias is the one you absolutely do not want to do. Giving Valygar's body to Tolgerias will result in you being permanently locked out of the Planar Sphere dungeon missing all of that EXP and all of that loot. There are some best in slot items in there so you absolutely do not want to do this.
Once Valygar is in your party or you have his body in your inventory you'll be able to open the Planar Sphere in the Slums. At this point you can technically kick him out of your group too and complete the dungeon by yourself; once you've used Valygar to open the door he is no longer required for anything.
Until you finish the Planar Sphere dungeon this quest will remain active in your log. For more information about the Planar Sphere I recommend you check out my Planar Sphere Overview guide or my Inside the Planar Sphere quest walkthrough.