You're a wretched imp/thrall/whatever in a monster army for an evil witch/demon queen and through some incredible feat of heroism or luck you manage to earn her notice and she rewards you with bigger and stronger forms and powers as you prove yourself useful, eventually leading to secks and/or love and taking over the world together. Maybe you'd have jealous rivals trying to sabotage your ladder climbing, or tempt you to join the good guys or something too idk.