El Dorado's Judgement (Recharges after a Short or Long Rest). If Yuvon'Diiv is reduced to 0 hit points while inside of his lair, he absorbs all of the gold in his lair and resets to 400 hit points, recharges his boiling breath, and regains all uses of its legendary resistances and radiant runes. He also gains a one-time use of his mythic action. Yuvon'Diiv also loses his cold vulnerability when this ability activates. 

Legendary Resistance (3/day). If Yuvon'Diiv fails a saving throw, he can choose to succeed instead.

Illumination. Yuvon'Diiv sheds dim light in a 30-foot radius.

Renew Gold.  While in its lair, Yuvon'Diiv heals 20 hit points per turn. This can be stopped by dealing 30 cold damage in a single turn. Damage is counted even if it is not of the ice element.

Gold Mirror. If Yuvon'Diiv takes radiant damage, each creature within 40 feet of it must make a Constitution saving throw (DC 18) or take full damage on a failure or half damage on a success. If the Molten Gold Core Worm is in its lair, targets take full damage on hit. 

Tunneler. Yuvon'Diiv can burrow through solid rock at half its burrowing speed and leaves a 20-foot-diameter tunnel in its wake.

All that Glitters. Yuvon'Diiv Can supernaturally sense any gold down to a singular gold piece within 1 mile. Cannot be surprised if any gold resides on any person or thing attempting to approach it. 

Radiant Runes. Yuvon'Diiv has advantage on saving throws against spells and other magical effects, and spell attacks have disadvantage against him. Yuvon'Diiv cannot be affected by Transmutation magic or its effects (including time stop). 

Multiattack. Yuvon'Diiv makes three attacks: two with its midas touch and one with its bite.

Midas Touch. Melee Weapon Attack: +15 to hit, reach 10 ft., one creature. Hit: 26 (6d6 + 14) piercing damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. Yuvon'Diiv can grapple two creatures at a time this way. The grappled creature(s) take 3d8 fire damage at the start of each of the worm’s turns. While restrained by the worm in this way, it consumes 500gp from the grappled creatures’ inventory.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 30 (5d8 + 14) piercing damage.

Breath Weapons (Recharge 5–6). Yuvon'Diiv uses its breath weapon.

Boiling Breath. Yuvon'Diiv exhales super-heated air in a 90-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 56 (12d8) fire damage on a failed save, or half as much damage on a successful one. The area hit by the breath weapon turns to glass for two turns, causing 12 (3d8) fire damage. 

Yuvon'Diiv can take 5 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. Yuvon'Diiv regains spent legendary actions at the start of its turn.

Midas Touch (1 action). Yuvon'Diiv can make a Midas Touch attack. 

High Roller (2 actions). Yuvon"Diiv slams its body into the ground and sweeps a 30 foot cone. Targets caught in this area must succeed a DC 21 Dexterity saving throw or take Hit: 26 (4d6 + 14) bludgeoning damage on a failure, half on a success.

Liquify sand (3 actions). Yuvon'Diiv vibrates its body and causes the sand around it in a 200-foot radius to become liquid, causing any targets to sink down to their waste on a failed DC 18 Strength saving throw. Targets who fail this save are restrained and must use an action to free themselves on their turn. 

In His Image (one-time use). Upon activating El Dorado's Judgement, Yuvon'Diiv spews molten gold out in a 200-foot cone. Any creature in that area must make a DC 24 Constitution saving throw. On a failed save, a creature takes 24d6 fire damage and is coated in molten gold and restrained. The gold can be removed with action from another creature. The fire damage caused by this attack bypasses resistances and immunities.