From D&D Wiki
A Gate Key is large magical Hammer built to resemble a key, with the bow of the key, being the hammer end. The Weapon is filled with a mysterious energy that makes this weapon exponentially powerful.
When attacking with this weapon, if your attack roll beats their touch AC it will deal Half Damage and and the target must make a DC 20 Fortitude Save or be knocked prone (Regardless if the attack roll did not beat their full AC). If this Weapon dealt damage to your opponent, your next attack will deal damage as if it were on size catagory higher.(This Effect Stacks.)
This Weapon on a successful Critical Hit, will Teleport the opponent as if by the Dimension Door spell, except that it will teleport them away to full distance of the ability and in a straight line backwards from the point of origin.