My idea for a game is to start with a group of low-level wizards who are drawn to a secret cult by cultists.
They don't know what the cultists want to do with them.
The cultists promise them strange new powers, but only if they willingly sign a binding contract.
Part of it entails that they have to eat strange food, which will taint their being and their magic ....
In the end they will have a new class, which I've made myself, based on the wizard -class.

I'm looking for advice on the following issues:
- on which occasion can they be lured to the cult by the cultists? What would be most logical?
- what do I do (as DM) if they refuse to eat the food?
- how can I get the players to cooperate while at the same time the cultists keep their own plans secret?
- the evolution into the new class can only succeed if they continue using their spells. What kind of adventures can I create for the low-level wizard group, so that their powers are challenged?

In reply to Youp (msg # 1):

How about do it in reverse.

Start the game after they have joined the cult and eaten the "strange" food.
Let each player supply the reasoning why the wizard chose to join and eat the food.

By making this part of the social contract between the all the players (you included) they all agree that joining the cult and taking part in odd rituals to gain power is part of the game.

If you play a D&D game, wizard as a class is pretty weak without magic. If you need to give the PCs more incentive to use magic just add a bonus XP for spell casting (maybe spell level*100 xp per casting while in an adventure).

Another option is shorten the refresh time, maybe spell slot regenerate after an hour or maybe they need to drink their enemy blood and regain spell slots?

Both these options can encourage players to use more magic.

I agree that starting the game after they ate the food is best; it just eliminates almost all of the problems you have.

As for the last question, I think the cult should offer everything that wizards lack or are weak in. Worried about your inability to wear armor? The cult offers a stronger version of mage armor that lasts longer. Wish you had more hit points? We offer the ability to have your hit points drop to 1 instead of -20 from that attack (only usable X # of times per day). Infinite spell shots for level zero spells, more spell slots, higher Carter level, whatever fits the theme of the taint. Offer them everything they could ever want and make them all overpowered... Then show them how evil the cult is. Will they turn against the cult, or stay evil?

How to challenge them? Put them against opponents who are resistant or immune to certain types/schools of magic.

Okay, thanks for your good suggestions!

This all sounds fascinating and I would be quick to RTJ if you create this game.

I would as well.

Wizards finding out that the source of their uberpowers is fantasy!Satan would be fun. Sign me up.

I'd be down to play this!