“Decayed Commander”’s card-discarding effect only activates when “Decayed Commander” attacks directly AND deals 1 or more points of Battle Damage to the opponent, so if the opponent uses “Kuriboh” or “Waboku” then “Decayed Commander”’s effect does not activate.
“Decayed Commander” does not activate its card-discarding effect if destroyed with “Ring of Destruction”, or if it attacks a face-down monster while equipped with “Fairy Meteor Crush”, or due to reflect damage from “Relinquished”, etc.