[Saber Cherry]

My recollection is that decay simply adds poison and fatigue to the commander every turn. Undead are immune to both, so decay won't affect them. Bane venom is described as causing a fast-acting disease, only undeads can safely carry Bane Venom Charms; Banes (the source of Bane Venom) are undead and the Lord of Banefires is undead, and neither of them are affected. So there are various precedents for Decay to only affect the living, and they agree with my memory of the mechanics.