Scáthach temporarily summons a massive gate to her shadow realm, and it sucks in all life forms in range. If the target fails to resist it with one's MGI or LCK stat, they're sucked into the gate and instantly killed. Scathach can choose specific targets on whether to allow them access or not. Even if resisted, the targets' mana is rapidly drained and they take major damage.
In the events of Fate/Grand Order, she attempts to forcibly send Cú Chulainn Alter to the Land of Shadows with it, but fails as he retaliates.