[Suzerain of Sheol]

There's no hard and fast rule. Asasiah was crazy powerful when she joined, possibly too powerful, but I have no shortage of horrible creatures to challenge anything short of God himself. :P

And items are just an aspect of the character's power. Mine doesn't use any, I just gave him stronger powers to compensate.

As it stands, Dante, Fee, and Dara are sort of still in the "mortal" tier as far as power goes, while Na'lsa and Shealtiel have a huge suite of supernatural powers to draw on -- Na'lsa's seem to involve more being absolutely impossible to destroy and consuming single targets for his own benefit, while Shealtiel's powers are more offensive and destructive in nature.

So, somewhere in-between there would be a good ballpark, I suppose.