I reached a blood pool in Dryford Ruins and since my character is cruel I can choose to sacrifice one of my party members to gain "power". My question is..... what do I  gain exactly :D ?

Permanent +1 Might increase.

1 stat and skill based on the companion you sacrifice.

Cool, I didn't know you could do that. Do the companions have special dialogues related to this choice? Do the sacrificed companion turn hostile and you have to defeat him first before ritually sacrificed him or do you just casually push them into the blood pool? What's the other companions' reaction? I hope someone would uploaded this part on youtube.

Wonder if you can sacrifice an inn adventurer to this. 

Unless you play Ironman: Save and try it! :D

http://forums.obsidian.net/topic/76809-why-wont-blood-pool-let-me-sacrifice-a-non-story-character/?hl=%2Bblood+%2Bpool