Keyword Ability. Any damage a Lethal unit deals to a minion is enough to kill it. (Remember that zero damage isn't damage at all, so it will need to do one or more damage.) For example, Pit Vipers can kill Amazon Warriors with a single strike for 1 damage. If the Amazon Warriors had their Shield Maidens nearby, then the Pit Vipers wouldn't actually kill the Amazon Warriors, because they'd do zero damage, which isn't actually damage at all. A spellcaster with the Lethal ability still casts Magic spells normally; the Magic spells themselves deal the damage. For example, a spellcaster with Lethal casts Firebolts, but the damage from Firebolts is not Lethal; Firebolts is dealing the damage, not the spellcaster. On the other hand, the damage from Poison Nova is Lethal, because the spell itself says it's lethal.