Menace Manual: Mutant Manticore

This fella gave my players are real run for their money.  They initially surprised his mates – two normal manticores – and they thought they had won until he turned up in response to the howls of his pride-mates.  He actually never managed to land a bite or a claw but he did manage to get off 3 irradiated tail spikes during the 7 rounds of combat with one more ready to go if he wasn’t felled.  In the end our rogue rolled a sneak attack with advantage got a double critical (two natural 20s) and disembowelled it.  This beast was really an experiment with the new 5e encounter building rules.  Having more (or less) than the standard four player party changes the XP budge significantly so you end up being able to throw much cooler CR 13 monsters at level 5 characters!

MUTANT MANTICORE 
Large monstrosity, lawful evil

Armour Class 19 (natural Armour)
Hit Points 192 (24d10 + 72)
Speed 30 ft., fly 50 ft. (hover)

Str 17 (+3) Dex 16 (+3) Con 17 (+3)
Int 10 (+0) Wis 12 (+1) Cha 8 (-1)

Damage Resistances bludgeoning, piercing, and slashing from non-magical weapons
Damage Immunities radiation
Senses darkvision 60ft., passive Perception 11
Languages Common
Challenge 13 (10,000)

Call the Pride (1/Day). The mutant manticore as a bonus action can roar for aide from its pride.  A number of normal manticores will arrive within 1d4 rounds for each attacking character.

Mutations. The mutated manticore bears both weakness and boons as a result of its condition.  Additionally it has a fractured mind that causes it to become confused for 1 round whenever it fails a DC 10 Intelligence saving throw which it must make each round.

Tail Spike Regrowth. The manticore has twenty-four tail spikes. Used spikes regrow when the manticore finishes a long rest.

LAIR ACTIONS
When fighting inside its nest, the manticore can perform an extra action. On initiative count 20 (losing initiative ties), while hovering over its nest, it causes a downdraft which whips coils of spiky wire and bits of sharp rubble into the air, causing any creature within 20 ft. to also take 1d4 points of piercing damage each round.

ACTIONS
Multiattack. The manticore can use its Frightful Presence.  It then makes three attacks: one with its bite and two with its claws or a single attack with its irradiated tail spikes when ready.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target.
Hit: 7 (1d8 + 3) piercing and radiation (DC 10 CON or 1 level of exhaustion) damage.

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft. , one target.
Hit: 7 (2d4 + 3) slashing damage.

Irradiated Tail Spike (Recharge 5-6). The manticore can shoot irradiated spikes in a 60-foot cone.  Each creature in that cone must make a DC18 Dexterity saving throw, taking 24 (6d8) piercing damage on a failed save, or half as much damage on a successful one.  A failed save also incurs mild radiation poisoning gaining the poisoned condition.

Frightful Presence. Each creature of the mutant’s choice that is within 120 feet of the manticore and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the dragon’s Frightful Presence for the next 24 hours.

Art credit for Manticore goes to IzzyMedrano.