Ok, let’s get back to fighting devils. How about a wizard that fights them by binding them?
School of Daemonologists
You view a hammer as a tool. You can build a church with one or you can kill with the same hammer. Demons and devils are just tools for you to use. Left unchecked they thirst for inocent blood and souls, but when properly controlled, you can build astounding things. They are your tools, and you are their wielder. Devils, demons, and the random daemon all fall under your toolbox, so you use them as you see fit.
Beginning when you select this school at 2nd level, you gain a familiar fiend, as per the find familiar spell, except the familiar must be a fiend and can be up to CR 1. As an action on your turn, you can directly command the fiend to take an action of your choice. When you take a move action, your fiend takes a move as part of that action.
Beginning at 2nd level, you gain the following spells into your spell book:Protection from Evil and Good, Hold Person, Banishment, and Dispel Evil and Good. Your wizard level must be an equal level to a cleric who is able to cast these spells for you to cast these spells, but your training has granted you access to these spells when you are ready.
Starting at 6th level, your bound servent can now be commanded by a bonus action and can be up to CR 2.
Beginning at 10th level, you’ve studied enough fiend names that now you can recall them as part of your spells. Any spell that specifies a creature type can now be applied to fiends.
Starting at 14th level, you pour power into your bound creature. It can now be a creature up to CR 4 and metamorphs into a CR 4 fiend of your choice. Furthermore, your find familiar spell now painfully carves your spellbook into the flesh of the fiend. You no longer need to carry your spellbook and can simply read the creature to prepare spells each day.