In BI, playing as the huns, I took the sassanid settlement first, while he had all family members inside.
Well, I won the siege and killed all the members, but as his town fell he just turned into a horde and spawned a few new family members (including faction leader)
So I guess that's what would happen with mongols/timurids too.

You have to take out all family members without taking his last town to really wipe em out. Or just take his town and then deal with the horde...