Cycle. Yidaro emits an aura with a 120 ft. radius that encourages magic surges. When rolling for wild magic, crystal rolls, or similar affect that would result in a magic surge, if the roll does not result in a magic surge, roll again. 

Hasty Start. During the first round of combat Yidaro's movement speed is doubled and it can make an additional attack.

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

Nigh Unkillable (3/day). If Yidaro would be reduced to 0 HP, Yidaro is stable. If Yidaro is at 0 HP and there is a spell cycle round, Yidaro is returned to full hit points.

Siege Monster. Yidaro deals double damage to objects and structures.

Trample. If Yidaro reduces a creatures HP to 0 the Yidaro may make an attack as a bonus action.

Multiattack. Yidaro makes two attacks: one with its slam, one with its tail.

Slam. Melee Weapon Attack: +16 to hit, reach 15ft., one target. Hit: 31 (4d10 + 9) bludgeoning damage and knocks the target 10 ft. away from Yidaro.

Tail. Melee Weapon Attack: +16 to hit, reach 20ft., one target. Hit: 41 (5d12 + 9) bludgeoning damage. If the target is a creature, it is pushed back 15 ft. away from Yidaro and  must succeed on a DC 24 Strength saving throw or be knocked prone.

Rejuvenation Cycle.  If a crystal roll has resulted in a spell cycle round for the fourth time in combat (or more), Yidaro recovers 200 hit points, it's speed is doubled for the round, and it can make an additional attack. This reaction expends one use of Nigh Unkillable.

Shell Defense.  Yidaro retracts into it's shell adding 4 to its AC against one attack that would hit.

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

Attack. Yidaro makes one melee weapon attack.

Charge. Yidaro moves up to its speed.

Tail Slam. Yidaro slams the ground with it's tail. All creatures within 14 ft. that are large or smaller must make a DC 24 strength saving throw or be knocked prone.