Elemental Nature. An elder elemental doesn’t require air, food, drink, or sleep.

   Heralds of the Storm. Xixecal are tremendously powerful elemental creatures that rise from the Elemental Plane of Ice. They conjure storms up to ten miles around them that are one part hurricane and another part blizzard. 

   Dragon’s Approach. White dragons are often drawn to the presence of a xixecal. Being immune to the cold and sharing the elementals' desire to see everything covered in ice, the dragons will protect the xixecal from anyone or anything that seeks to stop its march of devastation. They will sometimes nest on the xixecal’s back or shoulders, forming a sort of symbiotic relationship. There is no safer place for a white dragon than on the back of an elder ice elemental. 

   Ancient Aspects. The xixecal is an ancient and powerful being, harkening back to a time when the world was nothing more than chaos. These elementals have become so large and so detached from the plights of smaller beings, they are simply driven by instinct to cover the land in ice.

Frigid. A hit from a xixecal's melee attack induces numbing cold and forces the target to make a DC 20 Constitution saving throw. On a failed save, the target's speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can’t use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature’s abilities or magic items, it can’t make more than one melee or ranged attack during its turn. If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn’t take effect until the creature’s next turn, and the creature must use its action on that turn to complete the spell. If it can’t, the spell is wasted. The creature may attempt another saving throw at the end of each of its turns to end the effect.

Ice Walk. The xixecal can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.

Rend. If the xixecal hits with both claw attacks, it latches onto an opponent's body and tears the flesh. The second attack automatically deals an additional 18 (4d8) slashing damage.

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

Multiattack. The xixecal makes two claw attacks.

Claw. Melee Weapon Attack: +17 to hit, reach 15 ft., one creature. Hit: 32 (4d10 + 10) cold damage.

Biting Cold (Recharge 6). The xixecal unleashes a 300-foot cone of biting cold air. Each creature within the cone must make a DC 20 Constitution saving throw, taking 42 (12d6) cold damage on a failed save, or half as much on a successful one. If a target’s saving throw fails by 5 or more, the creature is also frozen solid. A creature who is frozen solid is unconscious and restrained in place until they take any amount of fire damage. The creature may also attempt a DC 25 Strength saving throw at the end of each of their turns to break free of the ice.

Enter the description for how bonus actions work for your monster here.

Legendary Actions The xixecal  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 xixecal  regains spent legendary actions at the start of their turn.

 Claw.  The xixecal makes a claw attack.

 Move.  The xixecal moves up to its speed. 

 Ice Wall (Costs 2 Actions.  The xixecal casts wall of ice.

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