Baag Olo's Insanity

Cycle of Rebirth. If the dragon dies, its soul coalesces into a steely egg and teleports to a random plane of existence. The egg is immune to all damage and hatches into a time dragon wyrmling after 1d100 years. The dragon retains all memories and knowledge it gained in its previous life.

Legendary Resistance (5/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Metallic Awakening (Recharges after a Short or Long Rest). If the greatwyrm would be reduced to 0 hit points, its current hit point total instead resets to 450 hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, the greatwyrm can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 120,000 XP (240,000 XP total) for defeating the greatwyrm after its Metallic Awakening activates.

Reflective Carapace. Any time the tarrasque is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the tarrasque is unaffected. On a 6, the tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the tarrasque, turning the caster into the target.

Siege Monster. The tarrasque deals double damage to objects and structures.

Magic Resistance. The tarrasque has advantage on saving throws against spells and other magical effects.

Unusual Nature. The greatwyrm doesn’t require food or drink.

Multiattack. The dragon on its turn makes three attacks and can use its Frightful Presence.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon 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 the dragon's Frightful Presence for the next 24 hours.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 13 (2d12) force damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 19 (2d8 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. The greatwyrm can have only one creature grappled in this way at a time.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 21 (2d10 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be knocked prone.

Rend. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 22 (3d8 + 9) slashing damage plus 10 (3d6) force damage.

Breath Weapon (Recharge 5–6). The greatwyrm uses one of the following breath weapons:

Time Breath (Recharge 5–6). The dragon exhales a wave of shimmering light in a 90-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 24 Constitution saving throw. On a failed save, a creature takes 52 (8d12) 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, it has the poisoned condition, and other creatures have resistance to all damage it deals. On a successful save, the 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.

Elemental Breath. The greatwyrm exhales elemental energy in a 300-foot cone. Each creature in that area must make a DC 25 Dexterity saving throw, taking 84 (13d12) cold damage on a failed save, or half as much damage on a successful one.

Sapping Breath. The greatwyrm exhales gas in a 300-foot cone. Each creature in that area must make a DC 25 Constitution saving throw. On a failed save, the creature falls unconscious for 1 minute. On a successful save, the creature has disadvantage on attack rolls and saving throws until the end of the greatwyrm’s next turn. An unconscious creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

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.

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).

In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

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 90 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 60 feet to an unoccupied space it can see.

The greatwyrm 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 greatwyrm regains spent legendary actions at the start of its turn.

Attack. The greatwyrm makes one Claw or Tail attack.

Wing Attack (Costs 2 Actions). The greatwyrm beats its wings. Each creature within 30 feet of it must succeed on a DC 26 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The greatwyrm can then fly up to half its flying speed.

If the greatwyrm’s Metallic Awakening trait has activated in the last hour, it can use the options below as legendary actions.

Bite. The greatwyrm makes one Bite attack.

Shattering Roar (Costs 2 Actions). The greatwyrm unleashes a magical roar. Each creature in a 120-foot-radius sphere centered on the greatwyrm must succeed on a DC 26 Constitution saving throw or take 19 (3d12) thunder damage and be incapacitated until the end of its next turn.