Staggering Critical (4e Feat)
From D&D Wiki
Staggering Critical [Arachnomancer]
Your critical hits sent your foes reeling.
Prerequisite: Ability to score a critical hit with a weapon group on the roll of 19-20
Benefit: Choose a weapon group for which you can score a critical hit on a roll of 19-20 or better. When you make a critical hit with a weapon of that group, the target of the attack is slowed until the end of your next turn.