I suggest that your card text be revised to:
Shield: Any attacking creature has a 25 % chance to attack its owner.
Just for clarification of grammar and to follow convention: (every in game shield has 'Shield:' before its description except Bone Wall).
Now, on to the card itself- I like the idea. It does promote a creatureless deck from entropy's part, and I wonder how that would affect the meta. Good choice in making an
version of the
and
shields while keeping it unique.