This spell creates an electrical discharge that begins as a single stroke of lightning, 2½ feet wide, commencing from the fingertips of the caster. Unlike a Lightning Bolt spell, Chain Lightning strikes one object or creature initially, then arcs to a series of other objects or creatures within range, losing energy with each jump.
The bolt initially inflicts 1d6 points of damage per level of the caster, to a maximum of 12d6 (half damage if the object or creature rolls a successful saving throw vs. spell). After the first strike, the lightning arcs to the next nearest object or creature. Each jump reduces the strength of the lightning by 1d6. Each creature or magical object hit receives a saving throw vs. spell. Success on this save results in the creature taking only half damage from the bolt.
The chain can strike as many times (including the first object or creature) as the spellcaster has levels, although each creature or object can be struck only once. Thus, a bolt cast by a 12th-level wizard can strike up to 12 times, causing less damage with each strike. The bolt continues to arc until it has struck the appropriate number of objects or creatures, until it strikes an object that grounds it (interconnecting iron bars of a large cell or cage, a large pool of liquid, etc.), or until there are no more objects or creatures to strike.