Ok, something like:
Tail-Fling?? (Su): A ni'iath's tail-barb can create a gravity-manipulation field similar to the reverse gravity spell. If a ni'iath scores a critical hit on an object or creature of up to Huge size, the target is flung 100 ft. away. The ni'iath can gravity fling the target in whatever direction it chooses; if no strategically desirable direction exists the ni'iath flings the target in a random direction. During its flight, the flung object or creature feels no effects of gravity. At the end of its flight, the target again becomes sensitive to gravity and may take falling damage. The effect ends if the flung target hits an obstacle before travelling for 100 ft, but both the flung victim and the obstacle take 1d6? bludgeoning damage.
Just need to work out the threat range (18-20 or 17-20?) and the bludgeoning damage (stick to 1d6?).