Card of the Day - Secret Assassin/Cloistered Sage/Hidden

I was inspired by the ladies on my favorite MTG podcast, Magic:the Amateuring, to fix the cards Camouflage and Illusionary Mask.

I think the best way to do it is to create a new mechanic that does what those cards are trying to do.

I present:  Hidden.

A card with Hidden enters the battlefield hidden. While you cast it, you hide it from your opponent, and you place this reminder token on top of it.

If a Hidden creature attacks, blocks, takes damage, activates an ability, or becomes targeted, you immediately reveal it.

That's it.

It's important to remember that a hidden creature does NOT count as a face-down creature, even if it is turned face down so the opponent can't see it. Face-down creatures have very specific rules that affect them, and they don't affect hidden creatures.

Here are some examples of cards that would have Hidden:

See if you guys can come up with some other cool things that can use Hidden well.

*Post edited to fix wording and add clarification.