Any damage dealt via the power of a creature with the poison keyword during a fight destroys the damaged creature. This occurs when the damage is successfully applied to the opposing creature.
Poison has no effect if all of the damage is prevented by armor or prevented by another ability—poison only resolves when one or more damage is successfully dealt.
Poison refers only to damage that would be dealt by the creature’s power, not by damage that is dealt by keywords or other card abilities.
Yes. Macis Asp's power is still the source of the damage being dealt, which means that Poison also applies to that damage.Source: Crucible Cast 25