Multiattack. The githyanki makes two silver greatsword attacks.

Silver Greatsword. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage plus 10 (3d6) psychic damage. This is a magic weapon attack. On a critical hit against a target in an astral body (as with the astral projection spell), the githyanki can cut the silvery cord that tethers the target to its material body, instead of dealing damage.