From The Heroes of Might and Magic III wiki

Class	Air Magic
Basic cost	24
Duration	Instant
Level	4th

Does lightning damage. Strikes one stack, then moves to the stack closest to it and deals half the damage the previous one received. If it is resisted after it has already struck at least one unit, it will move one to the next. Will only strike a stack once, and will die out prematurely if it can't find enough stacks.

Level	Effect
None	Strikes up to four stacks. First stack receives (Spell Power *40 + 25) damage
Basic	Cost reduced by 4
Advanced	Strikes up to five stacks. First stack receives (Spell Power *40 + 50) damage
Expert	First stack receives (Spell Power *40+100) damage

[edit] Heroes specializing in Chain Lightning

• Solmyr the Wizard

[edit] Units immune to its effects

• Earth/Magma Elementals

[edit] Units extra vulnerable to its effects

• Air/Storm Elementals

[edit] Artifacts that render units immune to Chain Lightning

• Pendant of Negativity

[edit] See also

• Lightning Bolt
• Titan's Lightning Bolt