Actions

Multiattack. The demon makes seven attacks: six with its longswords and one with its tail. It can only make up to three attacks against the same target in a turn

Longsword. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) slashing damage.

Tail. Melee Weapon Attack: +9 to hit, reach 10 ft., one creature. Hit: 15 (2d10 + 4) bludgeoning damage. If the target is Medium or smaller, it is grappled (escape DC 19). Until this grapple ends, the target is restrained, the demon can automatically hit the target with its tail, and the demon can't make tail attacks against other targets.

Bladewaltz (Recharge 5-6). The demon uses its Multiattack, but all attacks target the same creature.

Summon Greater Demon (Recharge 5-6). The demon summons another demon unders its command onto an empty space the demon can see. The creature rolls it's own initiative and joins the fight starting with the next round.

Teleport. The marilith magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.

Reactions

Parry. The demon adds 5 to its AC against one melee attack that would hit it. To do so, the demon must see the attacker and be wielding a melee weapon.

Invoke Fury. The demon forces one creature it can see that has missed it with a melee attack this turn to make a DC 20 Wisdom saving throw. On a failed save, that creature is charmed and has to spend it's next turn making moving towards and attacking a creature of the demons choice.