Anons I'm in the Underdark and about to free Nere from the rumble. I have arranged with the duergar pair nearby to kill him, I've found the explosives, etc. But when I blow the debris and Nere gets out battle starts and everyone gang up on me (Nere AND all the duergar). Aren't some of them supposed to help me fight Nere due to our arrangement or something?
What's been happening here?

Cause as it is, I should just secretly kill every duergar I see before the boom boom. Otherwise it's very difficult due to high numbers and equal levels.