Form of Madness. If Ystodus would be reduced to 0 hit points, its current hit point total instead resets to 300 hit points, it recharges its Presence, and it regains any expended uses of Legendary Resistance. Additionally, Ystodus can now use its Step of Madness bonus action during the next hour.  As well, Ystodus gains both forms of Shift of Madness, 2 additional Strike of Madness attacks, and its Other Worldly Chill ability.

Legendary Resistance (3/Day). When Ystodus fails a saving throw, it may choose to succeed instead.

Immutable Form. Ystodus is immune to any spell or effect that would alter its form.

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

Arms of Madness. 2 misty arms form below Ystodus's arms.  Granting the Arms of Madness attack.  If in Form of Madness, an additional 2 arms form above it's arms.

Wings of Madness. 6 misty wings protrude from Ystodus's back granting a fly speed of 120 ft. and the Wing Guard reaction.

Otherworldly Chill (Form of Madness only). When a creature starts its turn within 20 feet of Ystodus or hits it with a melee weapon attack, it must make a DC 25 Wisdom saving throw.  On a failed save, the creature 2d10 cold damage and 2d10 necrotic damage, or half as much on a successful save.

Multiattack. Ystodus uses Presence if available, then makes 1 Strike of Madness attack, 1 Life Siphon Strike attack and 1 Tail Spear attack (Normal form only).  If in Arms of Madness, it makes 2 Arms of Madness attacks.  If in Form of Madness it makes 3 Strike of Madness attacks, 1 Life Siphon Strike attack, and 2 Arms of Madness attacks.

Strike of Madness. Melee Weapon Attack: +19 to hit, reach 5 ft., 1 target. Hit: # (1d10 + 10) bludgeoning damage and # (2d10) cold damage.

Life Siphon Strike. Melee Weapon Attack: +19 to hit, reach 5 ft., 1 target. Hit: # (1d10 + 10) bludgeoning damage.  The target must make a DC 26 Constitution saving throw.  On a failed save, the creature takes 3d10 necrotic damage and it's hit point maximum is reduced by that much.  Ystodus receives healing equal to the necrotic damage dealt.

Tail Spear (Normal form only). Melee Weapon Attack: +19 to hit, reach 25 ft., 1 target. Hit: # (1d8+10) piercing damage and # (2d10) necrotic damage.  The target must make a DC 26 Strength saving throw.  On a failed save, the creature is grappled and restrained.  The creature is also moved to an adjacent open area 5 ft of Ystodus.  At the start of each of the creatures turns, it takes # (2d8) bludgeoning damage.  The creature may repeat the saving throw at the end of each of its turns.

Arms of Madness (Arms of Madness form only). Melee Weapon Attack: automatic hit, reach 10 ft., 1 target. Hit: 25 force damage.

Presence (Recharge 5-6). Ystodus chooses one of the following three Presence options. It may not use the same Presence option more than once per turn.

1.Frightful Presence. Each creature of the Ystodus’s choice within 120 feet of it and aware of it must succeed on a DC 25 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage of Ystodus is within line of sight, ending the effect on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to Ystodus’s Frightful Presence for the next 24 hours.

2.Maddening Presence. Each creature of the Ystodus’s choice within 120 feet of it and aware of it must succeed on a DC 25 Wisdom saving throw or roll on the table of short-term Madness effects and immediately suffer from the listed effect. A creature may repeat its save at the end of each of its turns, ending all effects from Maddening Presence on a success.

3.Suffocating Presence. Each creature of Ystodus’s choice within 120 feet of it and aware of it must succeed on a DC 25 Wisdom saving throw. On a failed save, the creature takes 16 (3d10) cold damage, or half damage on a successful save. Additionally, Ystodus becomes surrounded by a 60 foot radius of suffocating fog that reduces all light to dim light and creates difficult terrain. Any creature within the fog cannot breathe or speak. The fog recedes and the effects end at the beginning of the Ystodus’s next turn.

Cataclysm of Madness (Form of Madness only). If Ystodus is reduced to 100 hit points during Form of Madness, it can channel the power of Xoriat to create a cataclysmic explosion.  Ystodus spends the next three turns forgoing it's turn to channel power.  On the 3rd turn, a massive explosion erupts, dealing 20d20 damage to every creature and object within a 4 mile radius.  Any creature within 100 ft of Ystodus receives doubled damage instead.  All creatures reduced to 0 hit points this way is reduced to mist including all non-magical items they are carrying.

Shift of Madness. Ystodus can use it's bonus action to switch between it's Arms of Madness and Wings of Madness abilities.

Flurry of Madness. Ystodus can use it's bonus action to make 2 additional Strike of Madness attacks.

Step of Madness (Form of Madness only). Ystodus can use the Far Step spell with a duration of 1 hour.

Deflect Missiles. Ystodus can use it's reaction to deflect or catch the missile when it is hit by a ranged weapon attack. When it does so, the damage it takes from the attack is reduced by 1d10 +30. If it reduces damage to 0 and have a free hand,  Ystodus can make a ranged attack with a range of 60/120.

Deflect Missiles Attack. Ranged Weapon Attack: +19 to hit, range 60/120 ft., 1 target. Hit: # (1d10 + 10) piercing damage.

Wing Guard (Wings of Madness form only). Ystodus can use it's reaction to deflect a melee weapon attack against it.  When it does so, the damage it takes from the attack is reduced by 1d10 +30. If it reduces damage to 0, Ystodus can cause the attack to hit another creature within 5 ft of it.

Counter of Madness (Form of Madness only). Ystodus can use it's reaction to use the Counterspell spell at 9th level.

Ystodus can take 3 legendary actions, choosing from the options below.  Only one legendary action option can be used at a time and only at the end of another creature’s turn. Ystodus regains spent legendary actions at the start of its turn.

Life Siphon Strike (Costs 1 Actions). Ystodus can make 1 Life Siphon Strike attack.

Arms of Madness (Arms of Madness only) (Costs 2 Actions). Ystodus can make 1 Arms of Madness attack.

Maddening Rush (Wings of Madness only) (Costs 2 Actions). Ystodus can move 60 ft in any direction without provoking opportunity attacks.

Ystodus gains an additional 2 legendary actions and the following actions during Form of Madness.

Step of Madness (Costs 1 Actions). Ystodus can use Step of Madness.

Orb of Madness (Costs 1 Actions). Ystodus pulls from the orbs orbiting it.  The orb then forms into a spear of dark energy.  Roll 1d6 to determine the outcome.
1. Paralyzing Spear. The targeted creature must succeed on a DC 26 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
2. Slowing Spear. The targeted creature must succeed on a DC 26 Dexterity saving throw. On a failed save, the target’s speed is halved for 1 minute. In addition, the creature can’t take reactions, and it can take either an action or a bonus action on its turn, not both. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
3. Enervation Spear. The targeted creature must make a DC 26 Constitution saving throw, taking 36 (8d8) necrotic damage on a failed save, or half as much damage on a successful one.
4. Petrification Spear. The targeted creature must make a DC 26 Dexterity saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.
5. Disintegration Spear. If the target is a creature, it must succeed on a DC 26 Dexterity saving throw or take 45 (10d8) force damage. If this damage reduces the creature to 0 hit points, its body becomes a pile of fine gray dust.
If the target is a Large or smaller nonmagical object or creation of magical force, it is disintegrated without a saving throw. If the target is a Huge or larger object or creation of magical force, this ray disintegrates a 10-foot cube of it.
6. Death Spear. The targeted creature must succeed on a DC 26 Dexterity saving throw or take 55 (10d10) necrotic damage. The target dies if the ray reduces it to 0 hit points.

Charge of Madness (Replaces Maddening Rush - Costs 4 Actions). Ystodus charges in any direction in a straight line up to its fly speed without provoking opportunity attacks.  Any creature Ystodus moves within 10 ft of must make a DC 26 Dexterity saving throw.  On a failed save the creature takes 6d8 force damage, or half as much on a successful save.  Any creature within 5 ft of Ystodus at the end of Charge of Madness takes an additional 2d8 damage.