The type of effect is too powerful with the type of trigger. I am sure there are other aspects of Darkness that are more fitting for a pact than Nightfall.
The upgrade was way too much of an increase
x2 damage
x2 duration (your two turns)
for only +1
We (I'm the second card ideator) thought about other possible effects, but we thougt that steal is really OP, and Syphon Life isn't praticable at all. So we have only other 3 possible choises, or, maybe, four. First is to use a Nightmare, but it would be useless after the first creature per turn. Second is Liquid Vampire, but the Darkness Nimph has the same effect, and we wouldn't make a copy of any existing card. The third choise is Nightfall, that can be nice, but with a really powerful effect on Devourers. The last possibility (I think it now, so it could really stupid, and I haven't talked with my friend) is to make the pact steal mana when you play a creature... But I'm not sure that I want to see another quanta denial for the Darkness. We would mindstorming on it tomorrow, maybe something could change. Thanks for the vision! (And sorry for English)