Pick one of the first 3 different creatures in your library. Move it to the battlefield. Then, it deals X damage to your opponent's first creature, where X is its power.
For being an 18-cost Rare, I would expect it to deliver the current full Power value in damage.
Rabid Bite is a 9-cost Common from the same set that has the same damage-infliction wording.
edit: Tested with Rabid Bite, using Legion Conquistador (base 3/3), creature reinforced once and buffed by Ajani Unyielding's 2nd ability (+2/+2) and Overcome (+8/+8) to an on-the-field 16/16, and it inflicted 16 points of damage on the opponent's first creature.