Magic Resistance. The Stain has advantage on saving throws against spells and other magical effects.

Two Heads. The Stain has advantage on saving throws against being blinded, deafened, stunned, and knocked unconscious.

Multiattack. The Stain of Demogorgon makes two attacks: it can elect to make two tentacle attacks; one tentacle attacks plus one other attack (either demonic cackling, gaze or fetid cloud); or two gaze attacks at different targets (one beguiling gaze plus one insanity gaze).

Tentacle (x2). Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 3) bludgeoning damage, and the target must succeed on a DC 10 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if its hit point maximum is reduced to 0.

Gaze. The Stain turns its magical gaze toward one creature that it can see within 30 feet of it. That target must make a DC 13 Wisdom saving throw. Unless the target is incapacitated, it can avert its eyes to avoid the gaze and to automatically succeed on the save. If the target does so, it can’t see the Stain until the start of his next turn. If the target looks at him in the meantime, it must immediately make the save.

If the target fails the save, the target suffers one of the following effects at random for 1d4+1 rounds. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success:

1. Beguiling Gaze. The target is stunned until the start of the Stain’s next turn or until the Stain is no longer within line of sight.

2. Insanity Gaze. The target suffers the effect of the confusion spell without making a saving throw. The effect lasts until the start of the Stain’s next turn. The Stain doesn’t need to concentrate on the spell.

Demonic Cackling (recharge 5-6). The Stain fixates one one creature,  making eye contact and cavorting and cackling so maniacally that the victim must succeed on a DC 13 Wisdom saving throw or falls into fits of equally insane laughter  for 1d4+1 rounds. During this time, the victim must make a DC 14 Wisdom save or fall to the ground in laughter, success means they can perform any action at Disadvantage.

At the end of each of its turns, and each time it takes damage, the target can make another Wisdom saving throw. The target has Advantage on the saving throw if it's triggered by damage. On a success, the effect ends.

Fetid Cloud (recharge 6). A 10­-foot radius of disgusting green gas extends out from the Stain. The gas spreads around corners, and its area is lightly obscured. It lasts for 1 minute or until a strong wind disperses it. Any creature that starts its turn in that area must succeed on a DC 14 Constitution saving throw or be poisoned until the start of its next turn. While poisoned in this way, the target can take either an action or a bonus action on its turn, not both, and can't take reactions.