In some rare cases, the creature may have subtypes other than creature types before becoming enchanted with Darksteel Mutation
However, if it phases out with that triggered ability on the stack, it won't be exiled, and the delayed triggered ability won't trigger again during future end steps.
(It's also not an enchantment they can kill to get their dork back, by the way) But by discarding it for an effect, you net the effect as well as a powerful madness theft of something at instant speed that can change the tables very quickly
Unblockable adders "When deal combat damage..." creatures "When attacks and isn't blocked..." creatures Click here for a text version of this decklist Click here for my EDH Commander decks
Being slightly less powerful than one of the most feared cards of all time still makes you pretty damn good, it turns out
Believe it or not, we actually have both of those things in Standard right now