Sacrificial Pit – how to use it correctly?

Taking diplomacy from Kifre means giving up the speed aura in favor of fewer than 10 Furies and about 15 Aces. No thanks.
In my opinion, it's a completely useless building.

You can gain empathy through diplomacy, and that's actually useful for a Warlock-Chaos Knight, and it makes a noticeable difference. So, I wouldn't rule out the possibility of leveling up diplomacy at Kifre.