I'm curious about these cast-anything wizards. It sounds horribly dangerous.
Within the context of the Belgariad novels, it is but there are a lot of limits placed on them by the setting.
The Good Side wizards (and this whole stupid series if full of B&W morality) are mostly immortals apostles of a god from the dawn of time, and they're all working toward a cryptic prophecy to keep the Bad Side from winning and an evil god from controlling the universe forever, yadda yadda. The big exception is the POV character, who's the Chosen One of the first series and repeatedly pulls off magic that simply shouldn't be possible because the prophecy says so. Literally. It talks to him sometimes. It's pretty silly. Oh, and there's the mundane blacksmith guy who becomes a wizard after getting killed and brought back to life by a god, which is supposedly against the rules but was prophesied so it's all good.
The Bad Side has relatively large number of priests of the evil god (who's mostly napping till the last book) who can do the same "will stuff to happen" magic but for no adequately explained reason are individually much weaker than any of the Good Side wizards, so they have to operate collectively to provide effective opposition. They tend to be kind of factionalized and prone to backstabbing each other, which may explain why they're so weak compared to the literal undying ancients on the other Side, but nebulous "native talent" seems to be a big factor. Some folks' just have stronger will than others, and sometimes gods empower people who weren't able to do a thing beforehand. The rules are...mushy, to put it mildly.
If I'm making this sound kind of stupid, it is. Didn't stop it from selling a bajillion copies, and there's a whole direct sequel series that renders the first five books and the big climax and fulfilled prophecy almost meaningless which sold almost as well.
But what works in a book doesn't make a good game. For practical purposes, you're better off treating "will magic" like Force powers, or Traveller-style psionics, and add some shape-changing magic on top of it - the Belgariad chumps could easily assume natural animal forms (favoring one particular type like a wolf or an owl) and there's at least one incident of changing someone else (into a snake, specifically) against their will. That might have required the assistance of a god or two so turning a squad of soldiers into mice is probably off the menu. You can do some big stuff with enough will behind it (changing the weather on a regional scale by moving masses of air around was doable but took practice and some understanding of how weather patterns form), but PCs probably need some fairly low upper limits that slowly improve with time. Using this stuff all took some concentration but not enough to keep you from talking, travelling, or noticing that jerk with a sword coming toward you, and maintaining an effect (like "don't notice us sneaking through your territory") long term was wearying both mentally and physically - we're talking days or weeks, not combat rounds. Presumably a potent will-user can just not sleep for extended periods when they want, although it's not really called out. In a game you probably need something more restrictive - a power point currency to spend and recover, or tests to resist exhaustion like Shadowrun uses.
One thing's for sure, there's no equivalent to Replace King here. You can kill one through any of a multitude of techniques if they don't have magical protections, you might be able to temporarily control their mind to make them do what you want - like stepping down and putting you on the throne - but rewriting the universe so that everyone sees you as king and doesn't see any problem with that is too many details for a human mind to juggle.
You want that effect, go over to Dying Earth (which is much better written anyway), bind a sandestin and have them do it for you - and then don't be surprised when they twist your desire to make the experience as ironically unpleasant as possible until you reverse yourself. Or another archmage notices what you did and undoes it, or worse, blackmails you to keep quite about it around the other archmages because cripes, being a king is really petty and beneath you. Or some overworld patron of the king steps in, slaps your sandestin down for messing with a greater power and you wind up reincarnated as a muck-shoveller in the royal stables.
Vance would have made a much better GM than Eddings.
