News: Age of Conan - SpellWeaving Revealed @ IGN

Spellweaving's major benefit is that it drastically powers up your spell damage and adds various kinds of special effects, all of which increases in power the longer you're in spellweaving mode. The downside is spellweaving can be lethal to the caster. When the spellweaving state is first entered, your stamina bar (used for things like running and melee combos) begins to drain. Once that bar is depleted, the spellweaving state will then begin to drain health, so you'll need to carefully monitor what's going on or your character might keel over and be forced to respawn. Should you need to, it's easy to stop spellweaving with a simple press of a key.