Cho'gall

Superior Unarmored Defense. While you are not wearing any armor, your armor class equals 13 + your Dexterity modifier + your Constitution modifier.

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

Two Heads One Body. Cho'gall is an ogre with two heads attached to the same body. For the purpose of combat he counts as two creatures, Cho and Gall who have separate initiative and actions but share hit points. When rolling initiative, roll once for each head. For ability scores/modifiers Cho's is the first number followed by Gall's and separated by a "/".

All Brawn and Muscle. Cho is the head in charge of the main body and thus Cho'gall's movement. 

All Brains and Dark Magic. Gall is the head in charge of spellcasting. He is immune to Silence and Stun effects.

Spellcasting. Gall is a 13th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). Gall has following warlock spells prepared:
Cantrips (at will): Eldritch Blast, Dancing Lights, Minor Illusion, Thunderclap
1st-5th level (3 5th-level slots): Misty Step, Negative Energy Flood, Enervation, Shadow of Moil, Spirit of Death, Blight, Counterspell, Fear, Hypnotic Pattern, Hunger of Hadar, Armor of Agathys

Cho Actions.

Multiattack. Cho uses his Punch attack twice.

Punch. Melee Weapon Attack: +9 to hit, reach 10 ft., 1 target. Hit: 12 (2d8 + 4) bludgeoning damage. 

Surging Fist. Can only be used if Surging Fist Charge was used as a bonus action on the previous turn. Cho punches and charges forward 20 feet in a line, knocking all enemies he hits 5 feet to either side of the line of their choosing. All creatures in the line must make a DC 17 dexterity saving throw or take 40 (8d8 + 8) bludgeoning damage on a failure and are knocked prone. On a successful save, they take half damage and are not knocked prone.

Throw Rune Bombs. Ranged Attack: +9 to hit, range 30 feet, 3 separate targets. Hit: 10 (2d6 + 4) Bludgeoning damage. Cho throws three rune bombs at three separate targets. Hit or miss, the Rune bombs land in the closest space to Cho within 5 feet of each target. 

Gall Actions.

Spellcasting. Gall casts one of the spells detailed in his spellcasting list.

Shadowflame (Recharge 5-6). Gall fires a beam of shadowflame in a 30-foot line that is 5 feet wide. Each creature in that line must make a DC 17 Dexterity saving throw, taking 24 (6d8) fire damage on a failed save, or half as much damage on a successful one.

Cho Bonus Actions.

Aggressive. As a bonus action, the creature can move up to its speed toward a hostile creature it can see.

Surging Fist Charge. As a bonus action, Cho begins winding up his fist and readying it to be unleashed on his next turn.

Consuming Blaze. As a bonus action, Cho ignites the surrounding area in shadow flame dealing 4 (1d8) fire damage to all creatures within 5 feet of him. The fire lasts for one minute on each creature hit unless put out using an action. While a creature is ignited in this way, it takes an additional 4 (1d8) fire damage when damaged by any of Cho's attacks or actions. When a creature is dealt fire damage in this way, Cho heals for half the fire damage dealt. 

Gall Bonus Actions.

Runic Blast. As a bonus action, Gall detonates all of Cho's Rune Bombs within 120 feet of himself that have been thrown at least a turn ago, all creatures within 5 feet of a Rune Bomb must make a dexterity saving throw or take 24 (4d8 + 8) fire damage on a failure. On a successful save they take half damage. 

Shove. As a bonus action, Gall mentally shoves Cho 15 feet in any direction.

Legendary Resistance x2.  If the creature fails a saving throw, it can choose to succeed instead.

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. Spent legendary actions are regained at the start of each turn.

Runby. Cho moves up to his movement speed without provoking opportunity attacks and can use one Punch attack against a single target during his movement. 

Spellcasting (Costs 2 Actions).  Gall casts one of the spells detailed in his spellcasting list.

Upheaval (Costs 3 Actions). Cho causes the ground in a 60 foot cone to upheave towards him. All creatures of Cho's choice within range must make a DC 17 Strength saving throw or take 30 (6d8 + 6) bludgeoning damage and be pulled 30 feet closer to Cho. On a successful save, creatures take half damage and aren't pulled.

Twisting Nether (Costs 3 Actions). Gall enshrouds the area near himself with dark energy. Until the start of Gall's turn the area within 15 feet of him becomes difficult terrain. At the start of Gall's next turn, the area explodes dealing 30 (6d8 + 6) necrotic damage to all creatures within the effects radius.