Aberrant Host. The warlock's mind and body has been twisted by exposure to the far realms and is simply a host to a greater aberrant lifeform.
The warlock has advantage on checks made to maintain concentration as the body is only roughly connected to the parasitic mind that controls it. It also counts as both a humanoid and an aberration for the purposes of spells and magical effects.
When the warlock is reduced to 0 hit points, the aberrant parasite is only destroyed if the warlock's entire body is destroyed or if the warlock is killed by radiant or psychic damage. If the parasite is not destroyed and a creature with a brain touches the remains of the warlock within 1d6+1 days of the warlock's death, that creature is infected with the parasite if they fail on a DC 16 Constitution saving throw (make this roll in secret).
After 1d8+2 days the first symptoms manifest as a runny nose with thick mucus. After an additional 1d8+2 days symptoms progress to a severe headache, consistent cough, and yellow-green mucus; if the target fails a DC 16 Constitution saving throw at the beginning of this stage, they become ill enough to be bed ridden.
After an additional 1d8+2 days, the infected creature is now infectious to others; any creature with a brain that touches the infected creature or any of its body fluids must succeed on a DC 16 Constitution saving throw or become infected themselves. The initial infected creature is bed ridden regardless of constitution and all symptoms have seemingly disappeared except for thick, milky white mucus that continues to drain from the nose as well as the ears. If the creature fails a DC 18 Constitution saving throw at the beginning of this stage they fall into a coma and are permanently unconcious.
After an additional 1d6+1 days, the infected creature must make a DC 18 Constitution saving throw every 24 hours. On a save nothing occurs and they continue to make saving throws every 24 hours. On a failure, the parasite within the infected devours the infected creature's brain and kills it instantly. The creature is rapidly transformed into a new aberration. Creatures without an Intelligence, Wisdom, or Charisma score of 16 or above are transformed into a mindwitness if Intelligence, Wisdom, or Charisma is their highest ability score, otherwise they become a gibbering mouther. Creatures with an Intelligence, Wisdom, or Charisma score of 16 or higher are transformed into a new warlock of the Forgotten.
Casting detect evil and good or using similar magic on an infected target before they transform, or succeeding on a DC 19 Wisdom (Medicine) check, reveals the creature is infected with an unknown parasite, but not how to cure it. The use of greater restoration or greater spells or effects such as wish can cure the creature and destroy the parasite inside its body, however such knowledge is rare and difficult to learn; characters may need to make contact with entities of the far realms or powerful planar beings such as archdevils, demon princes, elemental lords, or gods in order to acquire this knowledge.
Innate Spellcasting. The warlock’s innate spellcasting ability is Charisma. It can innately cast the following spells (spell save DC 15), requiring no material components:
At will: detect magic, jump, levitate, mage armor (self only), speak with dead
1/day each: arcane gate, true seeing
Spellcasting. The warlock is a 14th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 15, +7 to hit with spell attacks). It regains its expended spell slots when it finishes a short or long rest. It knows the following warlock spells:
Cantrips (at will): chill touch, eldritch blast, guidance, mage hand, minor illusion, prestidigitation, shocking grasp
1st–5th level (3 5th-level slots): armor of Agathys, arms of Hadar, crown of madness, clairvoyance, contact other plane, detect thoughts, dimension door, dissonant whispers, Hunger of Hadar, summon greater aberration, vampiric touch
Whispering Aura. At the start of each of the warlock’s turns, each creature of its choice within 5 feet of it must succeed on a DC 15 Wisdom saving throw or take 10 (3d6) psychic damage, provided that the warlock isn’t incapacitated.
Multiattack. The warlock uses its chill touch, eldritch blast, or shocking grasp cantrips or casts a spell of 1st level or higher. It can also use its eldritch grasp attack as a bonus action.
Chill Touch. Ranged Spell Attack: +7 to hit, range 120 ft., one creature. Hit: 14 (3d8) necrotic damage. The target cannot regain hit points until the start of the warlock's next turn. An undead creature hit by this attack also has disadvantage on attack rolls against the warlock until the end of the warlock's next turn.
Eldritch Blast. Ranged Spell Attack: +7 to hit, range 120 ft. Hit: 10 (1d10+4) force damage. The warlock creates three beams when using this attack, making seperate attack and damage rolls for each beam. The warlock can direct the beams as it chooses, such as at one target or up to three different targets of its choice.
Eldritch Grasp. Melee Weapon Attack: +7 to hit, range 30 ft., one target. Hit: 9 (1d8 + 4) bludgeoning damage and the target must make a DC 15 Constitution saving throw, taking 11 (3d6) acid damage on a failure. If a target has a brain and fails the saving throw by 5 or more, it is also infected with an aberrant parasite (see the warlock's Aberrant Host ability).
Shocking Grasp. Melee Spell Attack: +7 to hit, reach 5 ft., one creature. Hit: 14 (3d8) lightning damage and the target is unable to take reactions until the start of its next turn. The warlock has advantage on this attack if the target is wearing armor made of metal.