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Source: Player's Handbook

Description: You can increase the area of your spells.

Prerequisites:

You can alter a burst, emanation, line, or spread shaped spell to increase its area. Any numeric measurements of the spell's area increase by 100%. For example, a fireball spell (which normally produces a 20-foot-radius spread) that is widened now fills a 40-footradius spread. A widened spell uses up a spell slot three levels higher than the spell's actual level. Spells that do not have an area of one of these four sorts are not affected by this feat.