Post originally by Bhikku at 2005-04-14 01:54:20
Converted from Phorums BB System

Here's an alternate synopsis: an Arcanum is like a skill; but instead of performing mundane tasks like bluffing people, it allows you to do magical things like the Jedi Mind Trick, or telekinesis.

Arcana (the plural of Arcanum) can be acquired by various feats; some are only available to the Adept class, others are available to anybody, but are constructed in such a way that they may be more useful to one role or another. The Adept-only feats also allow some Arcana to be used untrained.

The actual use of a given Arcanum usually depends on a roll, like any skill role. Depending on the feats and roles used to obtain the Arcanum, your roll may have ranks akin to a maxed-out in-class skill, a maxed-out cross-class skill, or an untrained skill. As noted in my other post here (sorry for that), some uses of Arcana have a DC, others are limited only by the subject's Save, or the amount of effect you can achieve at your level.

Some of these magical "skills" are limited by the need to save against Fatigue, as Ran pointed out, and using these too frequently will impose a penalty to the save. There's a few that can be used either normally or with fatigue - for instance, the Move Objects ability has DCs for various weights, and doesn't usually require a fatigue check, but by exerting yourself (and taking the fatigue check) you can double the weight.

It's also worth noting that you can use less ranks in the Arcanum than you actually have available - which is useful if you want to keep the Fatigue DC low. So, for instance, a high-level Adept might be able to drain a small lake with his Water Shaping arcanum, but it's awful darn tiring. If he wants to manipulate a bucket of water, though, he doesn't need to bend all his power on it; he can just amp down the ranks he's using in order to reduce the Fatigue DC as well.

And certain arcana also threaten to corrupt the user, the mechanics being very similar to Fatigue (so that a single arcana might be used in both safe and dangerous ways, etc). The effect of corruption is similar to the Taint mechanic seen in Oriental Adventures and Unearthed Arcana.

My favorite part of the system is the use of magic as a kind of skill - it's a different way of limiting its use that makes it feel a little more natural, a little more "part" of the character, than the Vancian system. The use of fatigue and corruption as additional balances helps it retain some of that strategic, resource-management element that keeps it different from ordinary skill usage - so your mage isn't just a glorified rogue. It strikes a nice balance.