Huge fiend (devil), lawful evil

Armor Class: 19 (natural armor, shield)

Hit Points: 256 (19d12 + 133)

Speed: 30 ft , fly: 50 ft , swim: 50 ft

STR

23 +6

DEX

17 +3

CON

22 +6

INT

19 +4

WIS

16 +3

CHA

23 +6

Skills: Deception +13, Intimidation +13, Perception +10

Damage Resistances: Bludgeoning, Piercing, and Slashing from nonmagical attacks that aren't silvered

Damage Immunities: Cold, Fire, Poison

Condition Immunities: Charmed, Exhaustion, Frightened, Poisoned, Prone

Senses: Truesight 120 ft., Passive Perception 20

Languages: All, telepathy 120 ft.

Challenge Rating: 20 ( 25,000 (33,000 in lair) XP)

Proficiency Bonus: +6

Actions

Multiattack. Geryon's three bodies can each make three attacks but is differently equipped. Each body can make one Tail attack and one Claw attack. The third attack is a Lash of Immolation attack, a Fireshield attack, or Claw attack, respectively.

Tail. Melee Weapon Attack: +12 to hit, reach 10 ft., one creature. Hit: 17 (2d10 + 6) bludgeoning damage. If the target is Medium or smaller, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, Geryon can automatically hit the target with its tail, and Geryon can't make tail attacks against other targets with that body.

Claw. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 20 (4d6 + 6) cold damage.

Lash of Immolation. Melee Weapon Attack: +13 to hit, reach 15 ft, one creature. Hit: 14 (3d4 + 7) Slashing + (3) 1d6 fire damage. When Geryon scores a critical hit with an attack using this whip, the target also has the restrained condition until the start of Geryon's next turn, as fiery bands lash around the target.

Fireshield. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 20 (4d6 + 6) bludgeoning damage + 7 (2d6) fire damage + 7 (2d6) piercing damage.

Call to Order (Recharge 5-6). Geryon possesses a unique Horn of Blasting named Tintamarre. Each use of the horn's magic has a 10% percent chance of causing the horn to explode. The explosion deals 10d6 fire damage to the blower and destroys the horn.

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

Legendary Actions

Legendary Action Uses: 3. Immediately after another creature's turn, Geryon can expend a use to take one of the following actions. Geryon regains all expended uses at the start of each of their turns.

Infernal Glare. Geryon targets one creature he can see within 60 feet of him. The target must succeed on a DC 20 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 Lash of Immolation attack.

Lair Actions

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

Banish. Geryon casts Banishment.

Chill Blast. Geryon causes a blast of cold to burst from the ground at a point it can see within 120 feet of it. The cold fills a cube, 10 feet on each side, centered on that point. Each creature in that area must succeed on a DC 20 Constitution saving throw or take 28 (8d6) cold damage.

Hateful Restraints. Geryon targets one creature he can see within 60 feet of them. The target must succeed on a DC 20 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, carrying over the normal cacophony of Pandemonium. Any creature that finishes a short or long rest in this area must succeed on a DC 20 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 Pandemonium.