Legendary Resistance (3/day). I'm pretty sure you know what a Legendary Resistance is.

Not Undead Fortitude. When the UHMG reaches 0 hit points, it dies in some really dramatic way such as falling into lava or off a cliff, even if the dramatic method makes no sense, such as the UHMG falling off a cliff when there was no cliff there. However, the UHMG is not dead, because he cannot be truly killed without a Wish spell, and by the time you have the Wish spell you could defeat this thing instantly regardless.

Unholy Death Screech of Deadly Death. The Jason releases an unholy screech that any creature that can hear can hear from 666 feet away. Anyone who hears it must make a DC 16 Constitution saving throw, taking 44 (8d8+8) Psychic damage regardless of the save so the save is useless. Okay no I lied, you still take the damage but if you fail the saving throw, you have 36 hours to live unless someone Greater Restorationates you. If you die in this way, you cannot be revived without the Resurrection spell. Or the Wish spell, obviously. The screech does not effect the UHMG and does not effect the UHMG, even if nobody cares, including the DM, so the UHMG is not effected by the UHMG'z UDSoDD (Unholy Death Screech of Deadly Death).

Stabby Stab. Melee Weapon Attack: +2652 to hit, reach 5 ft., one target. Hit: 1,000,000 Piercing damage. The target bleeds and is supremely dead, unless you do revive or something.

Choose Your Fate. The UHMG says "Choose your fate" in a strange language literally everyone can understand even though they've never heard of the language. The UHMG then holds out both hands, with a tiny skull in each one. The chooser must choose their fate. The skull represents death, and the skull represents death. If the chooser desires they can make a check of challenge 16 Perception, and on a success, they uncover the hidden third option, a skull that represents death. The skull is on a nearby table. If there is no nearby table, the skull is on a nearby table, which is nearby and is a table, and if there isn't a nearby table, there is a nearby table, which has a skull on it. The skull doesn't cause instant death, but it does cause death.

Mario. Jump high.