Class: (Un)limited Warlock

Years ago, I read a book about a demon and a wizard. I remember neither the name of the book nor anything but bits and pieces from the plot - the demon was trying to corrupt a young girl who could one day become a great force for either good or evil, the wizard was sent to stop the demon - but the way that magic worked for the wizard stuck with me.

The wizard wasn't an innate magic user, all of his magic was given to him and what he was given was everything he had. Several times throughout the book, he ended up in a situation where he really needed a spell, only for a flashback to reveal he had already used up that spell, years ago. Other times, he performed magic even though there was another way, and then a flashforward had shown us what kind of trouble this will bring to him in the future.

He had loads of power, but it was a finite, non-renewable resource.
 

Summoning Demons by Lukas Banas

 
Which brings me to the (un)limited warlocks.

Magic users tend to start weak, barely capable of casting a handful of cantrips each day, but then their power grows exponentially. Depending on your system, high level wizards might end up creating their own planes of existence and summoning dragons or angels to do their bidding, each and every day. But what if we turned this around?

Suppose that warlocks are given all their power by a patron. And by "all their power" I mean all of it - they get access to the whole spell list at level 1. However, they only get to cast each spell once, ever.

• In Vancian system, they have no spell slots and no spell level limit. Yes, they can use wish immediately at the start of the game.
• In GLOG, they have infinite MD and can put as many MD into a spell as they want (mishaps and dooms still apply).
• In mana point systems - well, I don't really have experience with those...

Your warlock player might absolutely obliterate the first boss with a single fireball - and that will also be the only fireball they will ever cast. They might trivialize the first set of obstacles - but what about the second one, or the third?

The (un)limited warlocks are all about self-restraint and adaptation to growing limitations. They start overpowered and grow weaker with each spell they cast and mark off their list. Soon, they will have to get creative, because they ran out of spells that would fully fit the situation. They will have to resort to the weird spells nobody bothers to take on most magic users. Even cantrips will become a scant resource.

And once their last spell is cast and gone, their pact will be due and their patron will collect their soul for purposes unknown. No save.

Is that balanced? Hell no!

Isn't that overpowered? Of course it is!

Is it playtested? Not at all.

Would it be fun? You tell me!

A patron and their warlock.
From Dark Souls 3

 
Oh, and if the player wants to get all clever with multiclassing into another magic user or learning new spells - that's a no go, friend. Your patron is keeping a very close eye on you and they will protect their investments with a vengeance, if necessary. After all, they already own your soul and can repossess it in case of your early death. It was only the contract you decided to breach that was preventing your sudden demise.