Legendary Resistance (3/Day).

If Geryon fails a saving throw, he can choose to succeed instead.

Magic Resistance.

Geryon has advantage on saving throws against spells and other magical effects.

Regeneration.

Geryon regains 20 hit points at the start of his turn. If he takes radiant damage, this trait doesn't function at the start of his next turn. Geryon dies only if he starts his turn with 0 hit points and doesn't regenerate.

Actions

Multiattack.

Geryon makes one Claw attack and one Stinger attack.

Claw.

Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 23 (4d6 + 9) cold damage. If the target is Large or smaller, it is grappled (DC 24), and it is restrained until the grapple ends. Geryon can grapple one creature at a time. If the target is already grappled by Geryon, the target takes an extra 27 (6d8) cold damage.

Stinger.

Melee Weapon Attack: +16 to hit, reach 20 ft., one creature. Hit: 14 (2d4 + 9) force damage, and the target must succeed on a DC 21 Constitution saving throw or take 13 (2d12) poison damage and become poisoned until it finishes a short or long rest. The target's hit point maximum is reduced by an amount equal to half the poison damage taken. This reduction lasts until the poisoned condition is removed. The target dies if its hit point maximum is reduced to 0.

Spellcasting.

Geryon casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 21):

At will: alter self (can become Medium when changing his appearance), detect magic, ice storm, invisibility (self only), locate object, suggestion, wall of ice
1/day: banishment

Teleport.

Geryon teleports, along with any equipment he is wearing and carrying, up to 120 feet to an unoccupied space he can see.

Legendary Actions

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

Infernal Glare. Geryon targets one creature he can see within 60 feet of him. The target must succeed on a DC 23 Wisdom saving throw or become frightened of Geryon until the end of its next turn.
Teleport. Geryon uses Teleport.
Swift Sting (Costs 2 Actions). Geryon makes one Stinger attack.

Lair Actions

On initiative count 20 (losing initiative ties), Geryon can take one of the following lair actions; he can't take the same lair action two rounds in a row:

Banish. Geryon casts the banishment spell.
Chill Blast. Geryon causes a blast of cold to burst from the ground at a point he can see within 120 feet of him. The cold fills a cube, 10 feet on each side, centered on that point. Each creature in that area must succeed on a DC 21 Constitution saving throw or take 28 (8d6) cold damage.
Hateful Restraints. Geryon targets one creature he can see within 60 feet of him. The target must succeed on a DC 21 Wisdom saving throw or become restrained for 1 minute. The target can end the effect on itself if it deals any damage to one or more of its allies.

Regional Effects

The region containing Geryon's lair is warped by his magic, creating one or more of the following effects:

Chilling Wind. Freezing strong winds howl around the area within 1 mile of the lair.
Enervating Screams. Howls and screams fill the air within 1 mile of the lair. Any creature that finishes a short or long rest in this area must succeed on a DC 21 Wisdom saving throw or derive no benefit from the rest.
Hellish Doorways. Sapient creatures within 1 mile of the lair frequently see shimmering portals leading to places they consider safe. Passing through a portal always deposits a traveler somewhere in Stygia.

If Geryon dies, these effects fade over the course of 1d10 days.