Agortthat the Spider Dragon

Legendary Resistance (4/ day) Agortthat can choose to succeed on a saving throw or ability check that it fails. 

Eyes of Umber. When a creature makes an attack roll on Agortthat, if they can see Agortthat's eyes, they must succeed on a DC 19 WIS saving throw or miss their attack roll. If they miss their attack roll by 5 or more, the attack is instead redirected at a nearby ally within range of the attack. Attacks made on allies by this effect cannot miss. 

Absolute Magical Resistance. Agortthat has advantage on ability checks and saving throws versus magical spells and effects. Spells of 3rd level or lower have no effect on Agortthat, fizzling against it's demonic hide. 

Misbegotten Anatomy. Agortthat can fit through any space that is at least 5 feet wide and tall, at half speed. It's body quickly twists and bends to allow it to fit almost effortlessly through nearly any space. 

Draconic Leech. When Agortthat kills a creature, it regains a number of hitpoints equal to that creatures CR (or level) + (10d4) + 10. 

Armaments of Agortthat. Agortthat the Spider Dragon is attuned to the Ruinblade (It is unaffected by the curse of the weapon), a Ring of Invisibility, and Infiltrator's Key in it's exalted form. 

Multiattack. Agortthat makes 5 attacks and uses it's Frightful Presence; Two with it's talons, two with it's arms, and one with it's bite. 
It can, instead, choose to make one Talons attack and one Breath attack. 

Talons. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 15 (3d6 + 8) slashing damage.

Arms (Ruinblade). Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 15 (5d6 + 8) slashing damage + 12 (4d8) force damage. 

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 19 (3d10 + 8) piercing damage plus 9 (4d8) acid damage.

Frightful Presence. Each creature of Agortthat's choice that is within 120 feet of it and aware of it must succeed on a DC 21 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Agortthat's Frightful Presence for the next 24 hours.

Spider-Breath Weapon (Recharge 5-6). Agortthat exhales a plume of tiny, venomous spiders from it's mouth in a 90 foot line which passes through walls that are less than 6 feet thick. These spiders shrivel and die shortly after their appearance, unless they are in a creature. Creatures within the line must succeed on a DC 21 dexterity saving throw, or suffer (25d6) poison damage, and become infested with spider hatchlings. 
A creature infested with spider hatchlings is considered Poisoned, and suffers (2d6) poison damage at the start of each of their turns. Additionally, their movement speed is halved, and at the start of every hour, their maximum HP is reduced by (2d6). Maximum HP lost by this effect takes 1 week to return for each time a creature has lost maximum HP. A creature that has their maximum HP reduced to 0 by this effects dies, their body convulsing and exploding, with a radius of 5 feet. Creatures within that radius must succeed on a DC 21 dexterity saving throw, or suffer (14d6) poison damage, and become infested with spider hatchlings.
A creature can only be cured of the infestation by a Greater Restoration spell, or by drinking the acidic blood of Agortthat - which subsequently deals (16d6) acid damage. 

Lolth's Embrace. Agortthat can use it's bonus action to vomit an acidic bundle of webbing at a target within 120 feet of it. The target must succeed on a DC 19 dexterity saving throw, or become grappled, restrained, and blinded. Additionally, at the start of each of the creatures turns, they suffer (2d12) acid damage. A creature can use their action to attempt a DC 19 Athletics check to break free of the acidic binding. 

Ruinblade. Agortthat can use either of the effects of the Ruinblade as a bonus action. The influence of Agortthat has permeated the weapon, increasing the DC of it's Blight effect to 20, and allowing the Disintegrate effects to target magical items as well as non-magical. 

Whiplash. Agortthat lashes out with it's lengthy, whip-like tail. A creature damaged by this attack is knocked Prone. Agortthat can use this reaction as well as an attack of opportunity between it's turns. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 15 (3d6 + 8) slashing damage.

Agortthat 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 Agortthat regains spent legendary actions at the start of its turn. 

Ruinblade (1 cost). Agortthat makes an Arms attack, or uses it's Ruinblade bonus action

Skitter (2 cost). Agortthat moves up to it's full speed in any direction it is capable of moving, without provoking attacks of opportunity. 

Inhale and Salivate (2 cost). Agortthat regains the use of it's breath weapon, and regains 50 hitpoints. The amount of hitpoints regained by this effect is reduced by 15 each subsequent use, until Agortthat's next long rest. 

In the Dark and Darker (3 cost). Agortthat unleashes an incredibly high-pitched screech. Creatures within 25 feet of Agortthat suffer (6d6) thunder damage, and must succeed on a DC 19 constitution saving throw, or become Stunned until the start of their next turn. A creature that was stunned by this effect cannot use actions on their subsequent turn after being Stunned. 
Creatures have advantage versus this saving throw if they have already attempted this saving throw within the last hour.