Make your big bad a grouping of liches/demons based on the seven sins.
Pride: The best magic user. There is NO ONE better than them. No one will EVER be better than them.
Envy: A man turned lich out of fear after his elven wife left him so she wouldn’t watch him die. He terrorizes all her lovers.
Gluttony: Consumed by their constant hunger from their former life they now devour the souls of lost and weary travelers and children.
Sloth: once a lazy king who ruled a great floating kingdom, he turned lich after he refused to put out the effort to stop a sorceress from destroying everything. She cursed him to a life of being bound by his soul to the very throne he would sink with. He saps life from the ocean without even lifting a finger.
Lust: A lich who lusts after bodies. They are constantly jumping from body to body. Trying to sate their lust for the next best looking body.
Greed: A lich who collects parts. He collects as many parts as he can to build horrible monsters in his lab. He will stop at nothing to get the best parts.
Wrath: A former man who died in his prime. Death turned him hateful and twisted. He believes that life is pointless and that death will always win out. He wishes greatly to see the flame of life sniffed out entirely.
(Disclaimer: these versions are just my ideas feel free to modify them and come up with your own!)