Creature: Corrupted Time Elemental (Eclipsos Eternum)

The theme of "corruption" returns as the players journey into the forests of Estalia, seeking the lost Elvern city. In this case, they stumble upon ruins controlled by a Time Elemental, Eclipsos Eternum, that has been driven mad in its attempts to stabilise the threshold given the magical "return" of the Elvern Kingdom.

Here are a few images;

Here are the stats I'm going to use for this unique individual;

Eclipsos Eternum, Corrupted Time Elemental

Large elemental, chaotic evil

Armour Class: 18 (natural armour)  
Hit Points: 225 (18d10 + 120)  
Speed: 0 ft., fly 60 ft. (hover)

STR: 20 (+5)  
DEX: 18 (+4)  
CON: 22 (+6)  
INT: 16 (+3)  
WIS: 14 (+2)  
CHA: 16 (+3)

Saving Throws: Dex +10, Con +12, Wis +8

Skills: Arcana +9, History +9, Perception +8  

Damage Resistances: acid, cold, fire, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical attacks  
Damage Immunities: necrotic, poison, psychic  
Condition Immunities: charmed, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, stunned

Senses: darkvision 120 ft., passive Perception 18  
Languages: Common, Primordial, telepathy 120 ft.  
Challenge: 18 (20,000 XP)  
Proficiency Bonus: +6

Time Distortion Aura:  

At the start of each of Eclipsos Eternum's turns, each creature within 30 feet of it must succeed on a DC 20 Wisdom saving throw or have their speed halved and take 14 (4d6) necrotic damage.

Actions:

Multiattack:  

Eclipsos Eternum makes two Temporal Slash attacks.

Temporal Slash:  

Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 25 (4d8 + 7) slashing damage plus 13 (3d8) necrotic damage. The target must succeed on a DC 20 Constitution saving throw or lose its reaction until the end of its next turn.

Chrono Burst (Recharge 5–6):  

Eclipsos Eternum releases a burst of temporal energy in a 30-foot radius. Each creature in that area must make a DC 20 Dexterity saving throw, taking 54 (12d8) necrotic damage on a failed save, or half as much damage on a successful one. Additionally, on a failed save, the creature ages or de-ages by 1d10 years (Eclipsos Eternum's choice).

Time Stop (1/Day):  

Eclipsos Eternum briefly halts the flow of time. As per the Time Stop spell, it lasts for 1d4 + 1 rounds instead of 1d4 + 1 turns.

Reactions:

Temporal Paradox:  

When hit by a melee attack, Eclipsos Eternum can force the attacker to make a DC 20 Wisdom saving throw. On a failed save, the attacker takes 27 (6d8) psychic damage and is stunned until the end of its next turn.

Legendary Actions:

Eclipsos Eternum 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. Eclipsos Eternum regains spent legendary actions at the start of its turn.

1. Temporal Strike: Eclipsos Eternum makes one Temporal Slash attack.
2. Disrupt Time (Costs 2 Actions): Eclipsos Eternum targets one creature it can see within 60 feet of it. The target must succeed on a DC 20 Wisdom saving throw or lose its next action as it becomes temporally displaced.
3. Rewind (Costs 3 Actions): Eclipsos Eternum regains 30 hit points.

These stats and abilities should make Eclipsos Eternum a formidable and memorable foe for your players to encounter!