Mysterious EggHow it works & interactions with other cards

How Mutation Triggers Work

The ability of Mysterious Egg triggers whenever a spell is cast for its mutate cost and resolves, resulting in it merging with the Egg. The ability triggers regardless of whether the mutating creature spell is placed on top of or underneath Mysterious Egg. Once the mutation is complete, you put a +1/+1 counter on the resulting permanent.

Resolving the Ability

When a mutate spell resolves, the permanent's 'Whenever this creature mutates' abilities are put onto the stack. If you have multiple mutation triggers, you can choose the order in which they resolve. The +1/+1 counter is placed on the merged permanent as a whole, meaning the bonus applies to the single creature that now shares all characteristics of its components.

Common Misconceptions

A common mistake is thinking that the Egg loses its counters if a new creature is placed on top. Because all components of a mutated creature are treated as a single object, all counters remain on that creature regardless of which card is currently the 'top' card. Additionally, note that if the mutate spell is countered or its target becomes illegal, the mutation does not occur and the Egg will not get a counter.

Strategic Timing

Since Mysterious Egg is an inexpensive early-game drop, it serves as a perfect 'base' for mutations. Because it doesn't have keyword abilities of its own, you generally want to mutate a creature with Flying, Trample, or Lifelink on top of it to ensure the +1/+1 counters are put to good use on a creature that can reliably bypass blockers.

Interaction examples

The Growing Construct

Walking Ballista

If you have Mysterious Egg on the battlefield and somehow turn Walking Ballista into a creature with mutate (or use a mutate spell on the Egg), the counters from both sources coexist. While Walking Ballista doesn't have mutate itself, if you mutate a third card onto a stack containing both, the Egg's ability will add a +1/+1 counter that can then be removed by the Ballista's ability to deal damage.

Mana and Mutation

Ornithopter of Paradise

If you cast a creature spell with mutate targeting Mysterious Egg and place it on top of the Egg, the creature gains the 'Whenever this creature mutates' ability. If you then mutate Ornithopter of Paradise (if it were granted mutate) onto that stack, the Egg ability triggers again. The resulting creature would have any +1/+1 counters previously placed by the Egg and the mana-generation ability of the Ornithopter.