Cluster spell (3.5e Feat)
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Cluster spell [Metamagic]
Any spell could be prepared as a supplement one. A supplement spell uses up a spell slot one level higher than the spell's actual level. Only a spell with area of affection could be prepared as a cluster spell. A cluster spell uses up a spell slot four levels higher than the spell's actual level. The time of casting is increases up to full-round action Any time you cast a cluster spell, you can immediately use any number of supplement spells in any place of clusters spell area instead of their range (you still can't use spells with target on you in any other creature, but you cold use touch spells on the creatures in the area). Supplement spell level (after applying metamagic feats) couldn't be higher than cluster spell level (after applying metamagic feats). A supplement spell could also be a cluster one.
On the other hand a few Hold person spells and fireballs in the acid fog could be too deadly for your party or its enemies and even one clever combination could win a battle in a single round. Also wizards could very fast find them without spells (that could a little bit solve first problem by making more than one battle or spell saving for a boss battle, that even could make fighters and rogues more interact in action and useful, but probably your party just reduce their 15 min adventurer day to 5 min).
About: this spell could make a lot of fun by making zigzags from line spells, fireball leaving a darkness and silence after exploding, healing mist and other endless possibilities of effects combination. Using a several spells at once is a bright show and even one clever combination could win a battle in a single round.