"As the clans carved out their territories, we saw allies where the Mardu saw only obstacles."
—Yasova Dragonclaw

Frost Walker

When this creature becomes the target of a spell or ability, sacrifice it.

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Fate Reforged

When Frost Walker becomes the target of a spell or ability, sacrifice it.

Friday Night Magic 2015

If Frost Walker becomes the target of a spell or ability, its ability triggers and is put on the stack on top of that spell or ability. Frost Walker’s ability will resolve (causing it to be sacrificed) first. Unless the spell or ability has another target, it won’t resolve as it tries to resolve for having no legal targets.