Monomolecular Blades. The edges of the Kaiser blades are so sharp, they are considered magical for the purposes of overcoming resistances and immunities to nonmagical attacks and damage. 

Frenzied. The Ultralisk is immune to the charmed, stunned and unconscious conditions. Also as a result of it's chaotic mind it is resistant to psychic damage.

One Track Mind. Any attempt to read the Ultralisk's mind or deal psychic damage to it causes a feedback discharge at the opponent, they must make a DC 17 Wisdom save or be stunned for one round.

Torrasque Strain (Recharges after a Long Rest). If the Ultralisk is killed, it enters a chrysalis state for 6 rounds, after which it regains all its hit points and becomes active again. The chrysalis has an AC of 18, 80 HP and a damage threshold of 15. and a 10 for each stat. If the chrysalis is destroyed, the Ultralisk also dies.

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

Multiattack.  The Ultralisk makes 2 attacks with its Kaiser blades.

Kaiser Blades. Melee Weapon Attack: +12 to hit, reach 10 ft., can hit up to 2 adjacent targets. Hit: 16 (2d10 + 7) slashing damage. 

Burrow Charge (Recharge 5-6). The Ultralisk quickly burrows and charges through the earth to a point within 40 feet of it, upon where it emerges. Each creature within 5 feet of the Ultralisk when it emerges must make a DC 18 Dexterity saving throw. On a failed save the creature takes 25 (5d10) bludgeoning damage and is pushed 10 feet away and knocked prone. On a successful save, the creature takes half as much damage and is pushed 10 feet away, but is not knocked prone.

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

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

Stomp. Each creature adjacent to the Ultralisk must succeed on a DC 18 Dexterity saving throw or be knocked prone.

Tissue Assimilation (Costs 2 Actions). The Ultralisk uses it's Kaiser Blades action. It regains hit points equal to half the damage dealt.