Beast of Hastur

Spider Climb: The Beast of Hastur can climb difficult surfaces, including upside down on the ceiling, without needing to make an ability check.

Telepathic Command: (Recharge 5-6) As a bonus action The Beast attempts to telepathically force a weaker-willed creature that it can see within 120 ft. of it to submit to it. The creature must succeed on a DC10 Wisdom Saving throw or be forced, on their next turn, to approach the Priest and take no actions which would harm it.

Amorphous: The creature can move through a space as narrow as 1 foot wide without squeezing.

Death Throws: When a Beast is reduced to 0 or fewer HP it will begin to writhe and flail about. On the next round it will attempt a final multiattack and Impregnate before dying.

Multiattack: The creature can make two attacks: one with its bite and one with its tentacles.

Bite: Melee Weapon Attack: +10 to hit, reach 5ft., one target. Hit: 14 (3d8 + 2) piercing damage plus 8 (2d8) poison damage.

Tentacles: Melee Weapon Attack: +10 to hit, reach 15ft., one target. Hit: 16 (2d12 + 2) bludgeoning damage. The target must make a successful DC 14 Dexterity or Strength saving throw or become grappled by the creature.

Swallow & Breed: (Grappled Target Only) Melee Weapon Attack: +10 to hit, reach 5ft., one target. Hit: 1 piercing damage. The target must make a successful DC 14 Strength saving throw or be swallowed. On the next round the creature must make a successful DC 14 Constitution saving throw or become infected with a parasite, at which point the Beast expels the creature, the parasite will burst from their chest turning them into a Minion of Hastur in 3 rounds. The parasite can only be removed via Lesser Restoration.

Impregnate: Any incapacitated creature may be impregnated by the Beast of Hastur as a reaction. A tentacle is inserted in a grappled creature's throat and deposits an egg. After one hour the impregnated creature must make a DC 15 Constitution save or the egg hatches and can control the impregnated creature at will.  An additional save it made at the end of each rest until 5 succecful saves have been made before 3 failed saves.

Success: The creature expels the egg and the eggs dies.

Failure: At this point the creature becomes a Minion of Hastur and learns all that the host creature knows. The creature retains its own consciousness and exists in perpetual agony and torture. The minion can errupt as reaction to more than 10hp or damage taken by the host or at will. After 1 week the Minion of Hastur evolves into a Spawn of Hastur, after 1 month the Spawn evolves into a Beast of Hastur 

Howl of Chaos: Bonus Action/Reaction (Recharge 5 - 6) The creature immediately howls on transformation, releasing a burst of of living chaos in all directions. All creatures within 60' must save vs sanity (DC 10 vs Sanity) or roll on the short term madness chart in the DMG. Any creature already affected by insanity caught in the howl suffers a -2 penalty to additional saves (stacking). Second failure, the creature affected rolls on the long term madness charts in the DMG. Third failure, the target must roll on the Permanent madness chart