Sans

Legendary Resistance (5/Day).  If Sans fails a saving throw, he can choose to succeed instead.

Meta Abilities and Knowledge. Sans can choose to succeed any save with a DC under 25. Sans knows that the fourth wall is there, and has knowledge of all timelines.  Sans is immune to any effect that would sense his emotions, change his memories or thoughts or read his thoughts, as well as any divination spell that it refuses. Sans can only fall asleep on his own accord.

Magic Weapons. Sans's weapon attacks are magical, and ignore all cover, or being ethereal.

Reactive. Sans can take one reaction on every turn in a combat.

Multiattack.  Sans takes up to 10 actions, but but with every initiative round that passes he takes an extra 10 for every passing initiative round.

Karmic Retribution.  Sans casts reverse gravity, but can also turn gravity sideways.  Sans can choose to be immune to the effects of this spell.

Shortcut.  Sans teleports to anywhere within the same plane of existence 

Gaster Blaster Overcharge (Recharge 5-6).  Sans unleashes a powerful Gaster Blaster attack in a line 10 feet wide and 100 feet long.  All creatures in the area must make a DC 20 Dexterity saving throw, taking 105 (30d6) force damage on a failed save, or half as much damage on a successful one.  Creatures in the area cannot benefit from force damage resistance or immunity.  

Gaster Blaster.  Sans summons a phantasmic skull that blasts a ray of energy at one target he can see.  The target must succeed on a DC 15 Dexterity saving throw or take 13 (4d8) force damage.  The target cannot benefit from force damage resistance or immunity.  

Bone Stab.  Bones erupt from the ground.  All creatures of Sans choice that he can see within 100 feet must make a DC 15 Dexterity saving throw, taking 5 (1d10) piercing and 5 (1d10) bludgeoning damage and being knocked prone on a failed save, or half as much damage and not being knocked prone on a successful one.

Bone Wall.  Sans creates a wall of skeletal parts 20 feet tall and 20 feet long.  Any creature that comes into contact with this wall takes 11 (2d20) force damage.  This wall cannot be broken or bypassed by any means.  It dissipates at the beginning of Sans's next turn.

You really think I was going to stand there and take it?  Sans takes zero damage this turn.

Sans 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. Sans regains spent legendary actions at the start of his turn.

Shortcut.  Sans can use Shortcut.

Change Gravity.  Sans can use Karmic Retribution.