Chain Lightning is a direct damage spell with a range of 24" that causes D6 Strength 5 hits with the Lightning Attacks special rule. Once the damage has been resolved, roll a D6; on a 2+, another enemy unit within 6" of the initial target also suffers D6 hits. Keep rolling for further enemies within 6" of the previous unit. Every time a new unit is struck the requirement increases by +1, so you require to roll a 3+ the second time, a 4+ the third time and so on. Proceed to keep rolling a D6 until you fail or there are no more viable targets. A unit can only be the target of Chain Lightning once per Magic phase.