Astralis, The Ageless Wanderer

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Multiattack. The dragon uses spellcasting and then makes three Rend attacks.

Rend. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 14(2d6 + 7) slashing damage plus 7 (2d6) force damage.

Time Breath (Recharge 5–6). The dragon exhales a wave of shimmering light in a 60-foot cone. Nonmagical objects and vegetation in that area that aren’t being worn or carried crumble to dust. Each creature in that area must make a DC 20 Constitution saving throw. On a failed save, a creature takes 36 (8d8) force damage and is magically weakened as it is desynchronized from the time stream. While the creature is in this state, attack rolls against it have advantage, and it has the poisoned condition. On a successful save, a creature takes half as much damage only. A weakened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself after it succeeds on three of these saves.

Time Gate (1/Day). The dragon conjures a 20-foot-diameter, circular portal in the space between its horns or in an unoccupied space it can see within 30 feet of itself. The portal links to a precise location on any plane of existence at a point in time up to 8,000 years from the present, whether past or future. The portal lasts for 24 hours or until the dragon’s concentration ends (as if concentrating on a spell). The portal has a front and a back on each plane where it appears. Travel through the portal is possible only by moving through its front. Anything that does so is transported to the destination, appearing in the unoccupied space nearest to the portal. Deities and other planar rulers can prevent portals created by the dragon from opening in the rulers’ presence or anywhere within their domains.

Spellcasting. The dragon casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (+12 to hit, spell save DC 20):

At will: guidance, thaumaturgy, true strike

1/day each: haste, slow, temporal shunt, time ravage, time stop

Change Shape. The dragon magically transforms into any creature that is Medium or Small, while retaining its game statistics (other than its size). This transformation ends if the dragon is reduced to 0 hit points or uses a bonus action to end it.

The dragon can take up to three reactions per round but only one per turn.

Reactive Rend. After using Legendary Resistance or in response to being hit by an attack roll, the dragon makes one Rend attack.

Slow Time. Immediately after a creature the dragon can see ends its turn, the dragon targets a creature it can see within 60 feet of itself that is weakened by its Time Breath. Until the weakened effect ends on the target, its speed becomes 0, and its speed can’t increase.

Time Slip. The dragon halves the damage it takes from an attack made against it, provided it can see the attacker. The dragon can then immediately teleport, along with any equipment it is wearing or carrying, up to 30 feet to an unoccupied space it can see.