Card name: Ambush!
Card Type: Enchantment
School: Nature
Level: 3
Hidden Cost: 2
Reveal Cost: X
Range: 0-2 Zone
Play this card face Down in a zone. This card cannot be play in a zone already occupied by an enemy creature.
When an enemy creature enters the zone this card occupies you must pay the reveal cost or destroy this card. This card can only be revealed this way. When this card is revealed search your spellbook for a creature and play it in this zone with its action marker down. The creature you play may make an immediate quick attack against the triggering enemy creature.
X = Creature's mana cost + Creature's Level x2
Thoughts?
(For the X cost I was also thinking about the costs listed on the Incantation Sleep instead).
(Also if the mage can no longer afford X, he can choose to pay the creatures level to just do the quick attack, and then return the creature to their spellbook or discard it, what do you think?)