I think it was 4 penetration bonus (spell focus +2, ring of sorcery +1, ring of wizardry +1). The spell resulted in only one of your strong leader (vampire lord, I would have prefered the dreadful Igarak) being enslaved.
That doesn't seem overpowered to me. Maybe you have been a bit unlucky.
And you killed this evil fountain (who was dumb enough to burn all of its astral gem in the enslavement and so failed to cast its returning spell
)