Dungeon Master

Omnipresent. The dungeon master can breath, move and speak unimpeded in any natural environment.

Shapechanger. As a free action the dungeon master can change its appearance to that of any other creature though its abilities, statistics and hit points remain unchanged.

Summon Minions (5/day). As an action the dungeon master summons monsters of a combined CR of no more than 10. These minions act independently of the dungeon master, but follow its will as long as the dungeon master is not incapacitated. If the dungeon master becomes incapacitated its minions view all creatures other than themselves as hostile and act accordingly.

Erase Rules (3/Day): The dungeon master uses it's action to remove one spell (level 5 or lower) or class feature or racial ability from the world until the next dawn. No creature/character can use that spell or benefit from the ability while it has been erased. Class features which appear in the class levelling table (e.g. Rogue's sneak attack) are immune to being erased.

Legendary Resistance (5/Day). If the dungeon master fails a saving throw, it can choose to succeed instead.

Spellcasting. The dungeon master is a 20-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +15 to hit with spell attacks). The dungeon master has following spells prepared:
Cantrips (at will): chill touch, spare the dying, vicious mockery 
1st level (4 slots):  healing word, command, feather fall

2nd level (3 slots):  alter self, calm emotions, misty step

3rd level (3 slots):  bestow curse, slow, haste

4th level (3 slots):  banishment, greater invisibility, fire shield

5th level (3 slots):  wall of force, telekinesis, animate objects

6th level (2 slots):  disintegrate,  guards and wards

7th level (2 slots):  reverse gravity, teleport

8th level (1 slots):  illusory dragon, antimagic field

9th level (1 slots):  meteor swarm

Multiattack. The dungeon master makes two swash attacks or uses its DMs Ire.

Swash. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. The target must make a DC 20 Strength saving throw, on a failure the creature is grappled and knocked prone. No more than two creatures can be grappled in this way at a time.

DMs Ire.  The dungeon master targets one creature that it can see within 500ft of it. Rock appear and fall on top of the target dealing 47 (5d10 + 20). The target must make a DC 20 Dexterity saving throw, on a failure the creature is buried becoming blinded and restrained by the rocks, on a success the creature takes half damage and is not buried. Buried creatures may use an action to free itself by making a DC 20 Strength (Athletics) check on their turn. Another creature may free the buried creature by succeeding on a DC 15 Strength (Athletics) check.

Blink. When the dungeon master is hit by an attack it can use its reaction to teleport up to 10 feet away causing the attack to miss. The dungeon master cannot use this ability if its movement speed is 0, creatures grappled to or by the dungeon master are transported with it.

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

Swash. The dungeon master makes one swash attack.

Command. The dungeon master gives an action to one of its summoned minions which that minion uses immediately.

Cast cantrip. The dungeon master casts a cantrip.

Cast spell (Costs 2 Actions).  The dungeon master can cast a spell using a spell slot as normal.