Legendary Resistance (3/Day). If the Abyssal Titan fails a saving throw, it can choose to succeed instead.

Innate Magic. The titan's attacks are all magical.

Spellcasting. The Abyssal Titan is a 20-level spellcaster. Its spellcasting ability is Charisma (spell save DC 26, +12 to hit with spell attacks). The Abyssal Titan has following warlock spells prepared:
Cantrips (at will): Soul Drain, Eldritch Blast, Demonic Visage  
1st level (14 slots): Armour of Agathys, Hex, Tenacity, Command                                          2nd level (12 slots): Hold Person, Ray of Enfeeblement, Spider Climb, Shatter                    3rd level (10 slots): Counter Spell, Fireball, Hunger of Hadar, Vampiric Touch                      4th level (8 slots): Blight, Wall of Fire, Fire Shield                                                                      5th level (6 slots): Hold Monster, Scrying                                                                                      6th level (4 slots)                                                                                                                              7th level (2 slots)                                                                                                                              8th level (1 slot)                                                                                                                                                                                                                                           

Multiattack. The Abyssal Titan can make 2 claw attacks and 1 bite/stomp attack as its action or make 2 quill attacks as its action instead.

Claw Attack. Melee Weapon Attack: +12 to hit, reach 15 ft., 1 target. Hit: 34 (4d12 + 10) slashing damage. 

Bite Attack. Melee Weapon Attack: +25 to hit, reach 5 ft., 1 grappled target. Hit: 80 (10d12 + 20) slashing damage and 30 (4d10 + 10) bludgeoning damage.

Stomp attack. Melee Weapon Attack: +10 to hit, reach 15 ft., 1 target. Hit: 30 (4d12 + 6) bludgeoning damage.

Quill attack. Ranged Weapon Attack: +10 to hit, range 50/200 ft., 5 ft. radius sphere target. Hit: 36 (6d8 + 12) slashing damage and 8 (2d8) poison damage.

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

Rage (Costs 1 Action). The titan fights with an unmatched fury and enters a fit of rage.

While raging, the titan gains the following benefits:

The titan has advantage on strength rolls and saving throws. Whilst the titan is raging all of it's attacks deal an additional 14 (3d6 + 5) fire damage, and it gains an aura of flame in a 10 ft. area around it, creatures in this area take 3 (1d6) fire damage if it moves into it during its turn and if it ends its turn in the aura.
 Rage lasts for 1 minute. 

Spellslinger (Costs 2 Actions). The titan can use a legendary action to cast any spell in it's spell list.

Titanic Tremor (Costs 3 Actions). The titan slams both fists into the ground in a fit of rage. All creatures within 200 ft. of the titan must make a DC 18 dexterity saving throw, on a failure the creature is knocked prone and takes 2 (1d4) bludgeoning damage, on a success the creature is unaffected. The surrounding terrain for 200 ft becomes difficult terrain.