Chain Lightning deals 3 damage to any target. Then that player or that permanent's controller may pay. If the player does, they may copy this spell and may choose a new target for that copy.
10/4/2004 The copy is a spell, so it can be targeted by other spells and abilities that target a spell.
10/4/2004 The player putting the copy spell on the stack controls that spell.
10/4/2004 The spell copy has all the text and can result in itself being copied.
6/8/2016 As Chain Lightning resolves, the targeted player or the controller of the targeted creature may copy it. The copy has the same text, target, and color of the resolving spell, though the player who’s copying it may choose a new target for it. Once that copy is created (or not), the first Chain Lightning has finished resolving and leaves the stack.
6/8/2016 The player putting the copy of the spell on the stack controls that copy.
6/8/2016 The copy of Chain Lightning is created on the stack, so it’s not cast. Abilities that trigger when a player casts a spell won’t trigger. Players may respond to that spell before it resolves.
6/8/2016 If the targeted permanent or player is an illegal target when Chain Lightning tries to resolve, the spell doesn’t resolve. It can’t be copied.