We've been here before, when I statted this utter horror up for 4th Edition. And here we are again, looking at the pure lethality that is the Umbral Blot. This version includes a few flourishes I've added, as well as some world specific bits and bobs (Sardai is a language created by the Settari, that is automatically understood by those who's genetic material contains their code at some level, and Settari Crysteel is a super hard, crystalline material they made weapons and armour from).
This thing is ridiculously tough, and only the strongest and most well organised groups are going to stand even a chance against it. Woebetide those that try to battle one of these without such strength...
If you wanted to dampen this things' viciousness a bit, you might want to rule that legendary weapons, or those made from Settari Crysteel (or its equivalent) are either immune to the Disintegrating Shell property, or at least have advantage on their saving throws. You might also want to change its damage from Bane damage to Force damage. In either case, I would reduce its CR to 23.
Umbral Blot – Medium Construct
A.C. 28 (Natural Armour)
Hpts: 250 (20d8+160) [180 - 320]
Speed: 30', Fly 90' (hover), Teleport 500'
Proficiency Bonus: +7
Str: 20 (+5) Dex: 30 (+10) Con: 26 (+8) Int: 23 (+6) Wis: 29 (+9) Cha: 30 (+10)
Skills: History +13, Perception +16
Damage Resistances: Acid, Cold, Fire, Lightning, Radiant; Ineffable Might
Damage Immunity: Force, Necrotic, Poison, Psychic; Slashing, Piercing and Bludgeoning damage from Non-magical, Uncommon, Rare and Very Rare weapons not made from Settari Crysteel
Condition Immunity: Poisoned, Blinded, Deafened, Exhausted, Diseased, Unconscious, Prone, Paralysed, Stunned; 150% Fortification, Immutable Form
Senses: Blindsight 500'; Passive Perception: 26
CR: 24 (62,000 xp)
Gravitational Lensing: Ranged attacks have disadvantage on attack rolls against the Blot due to the distortions of light around it.
Impossible Reflexes: The Blot adds twice its proficiency bonus to their initiative check.
Ineffable Might: The Blot reduces even Bane damage inflicted on it by 8 (Constitution modifer).
Legendary Resistance (3x/day): When the Blot fails a saving throw, it can choose to succeed instead.
Disintegrating Shell: Anyone hitting the Blot must make a D.C. 23 Constitution save or suffer damage as if hit by its Annihilating Touch. If a weapon hits it, the bearer must make a Dexterity save, or it is destroyed irrevocably. Unattended objects suffer 40d10 Bane damage per round of contact.
Immutable Form: The Blot is immune to all spells and effects that would alter its physical form. This includes petrification, polymorph and similar effects.
Spell Resistance: The Blot has advantage on saving throws against spells and spell like effects.
Multiattack: Blot makes two Annihilating Touch attacks
Annihilating Touch: Melee attack, Reach 5', One Target, +17 to hit; Hit: 110 (20d10) Bane damage. Targets that are reduced to 0 hit points by this attack are irrevocably gone; Critical Hit: Target must make a D.C. 23 Constitution saving throw or be destroyed as if reduced to 0 hit points.
Planar Travel: The Blot teleports to another plane of existence, having total control over where it arrives.
Gravitational Vortex (Recharge 5-6): All creatures within 30' of the Blot must make a D.C. 23 Strength or Dexterity saving throw, or be pulled adjacent to the Blot. Failed Save 5+: D.C. 23 Strength or Dexterity saving throw or touch Blot, suffering damage as if hit by its Annihilating Touch attack
WARNING - DO NOT TOUCH THE SPHERE OF UTTER DARKNESS