Amenderan The Great Fiend

Fire Aura. At the start of each of the The Great Fiend's turns, each creature within 5 feet of it takes 10 (3d6) fire damage, and flammable objects in the aura that aren't being worn or carried ignite. A creature that touches the The Great Fiend's or hits it with a melee attack while within 10 feet of it takes 20 (5d6) fire damage.

Magic Resistance. The The Great Fiend's has advantage on saving throws against spells and other magical effects.

Magic Weapons. The The Great Fiend's weapon attacks are magical.

Multiattack. The The Great Fiend makes three attacks: two with its Greatsword and one with its whip.

Greatsword. Melee Weapon Attack: +23 to hit, reach 15 ft., one target. Hit: 21 (3d8 + 8) slashing damage plus 50 (10d8) lightning damage. If the The Great Fiend scores a critical hit, it rolls damage dice three times, instead of twice.

Whip. Melee Weapon Attack: +24 to hit, reach 100 ft., one target. Hit: 11 (1d6 + 8) slashing damage plus 20 (5d6) fire damage, and the target must succeed on a DC 20 Strength saving throw or be pulled up to 25 feet toward the The Great Fiend.

Teleport. The The Great Fiend magically teleports, along with any equipment it is wearing or carrying, up to 150 feet to an unoccupied space it can see.

This creature has 2 legendary actions.

Wreath of flame. This creature creates a blazing fire engulfing 1 target. this target takes 40 (10d6) fire damage, and has disadvantaged on perception checks for the next minute.

Great slash. This creature makes 1 Greatsword slash against any creature within range.

Once this creature drops below 100 hp it regains all hit points and gains 1 additional legendary action.

You watch as it screeches out in pain as infernal runes glow along this creatures body. Then you see it, chains of flaming light, holy in nature form around its limbs. You see them binding it back, pulling it into the earth. Then it stops defending, as the ground crake and burns, you see the chains straining to hold it down as this creature fights for its freedom. Then there's a defining crash and the sound of chains breaking as its chains shatter into pieces. You see infernal runes glow along its body, and its eyes turn upon you all.

Infernal calling. Amenderan calls out summoning 1d4 demons of its choice with a CR lower than 15, all hominoids with an int higher than 10 within 100ft of it must make a will save or be frightened and paralyzed for 1d4 rounds.