Breadhemoth

Antimagic Cone. The  Breadhemoth's eye creates an area of antimagic, as in the antimagic field spell, in a 150-foot cone. At the start of each of its turns, it decides which way the cone faces. The cone doesn't function while the eye is closed or while the  Breadhemoth is blinded.

Demiplanar Donjon. Anything the Breadhemoth swallows is transported to a demi plane that can be entered by no other means except a wish spell or theBreadhemoth's Bite and Donjon Visit. A creature can leave the demi plane only by using magic that enables planar travel, such as the plane shift spell. The demiplane resembles a stone cave roughly 1,000 feet in diameter with a ceiling 100 feet high. Like a stomach, it contains the remains of past meals. The Breadhemoth can't be harmed from within the demi plane. If the Breadhemoth dies, the demiplane disappears, and everything inside it appears around the Breadhemoth's corpse. The demiplane is otherwise indestructible.

Unusual Nature. The Breadhemoth doesn't require air, food, drink, or sleep.

Spellcasting. Breadhemoth is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). Breadhemoth has the following wizard spells prepared:

Cantrips: infestation, toll the dead, mind sliver, mage hand, prestidigitation, ray of frost

1st level (4 slots): detect magic, magic missile, shield, thunderwave

2nd level (3 slots): blur, blindness/deafness, invisibility, mirror image, acid arrow, detect thoughts,

3rd level (3 slots): counterspell, dispel magic, fireball, slow,  animate dead, counterspell

4th level (3 slots): fire shield, dimension door, blight

5th level (3 slots): cone of cold, bigby's hand, cloudkill, scrying

6th level (1 slot): otto's irresistable dance, globe of invulnerability, disintegrate

7th level (1 slot): reverse gravity, plane shift, finger of death

8th level (1 slot): feeblemind, maze,  dominate monster, power word stun

9th level (1 slot): prismatic wall,  power word kill

Turn Resistance: The Breadhemoth has advantage on saving throws against any effect that turns undead.

Multiattack. The Breadhemoth can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 35 (4d10 + 14) piercing, psychic damage plus 14 (4d10) psychic damage.

Frightful Presence. Each creature of the Breadhemoth's choice that is within 150 feet of the Breadhemoth and aware of it must succeed on a DC 25 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Breadhemoth's Frightful Presence for the next 10 hours.

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

Light to doom. The Breadhemoth can take a reaction to replace any healing spells with poison. The person healing has to make a constitution saving throw of 20 Hit take (5d6+10) poison damage and the healing spell fails.

Summon mini Breads (Costs 2 Actions). The Breadhemoth can use an action to try to summon 1d4 beholder Bread, 1d10 ogre Bread, or 1d20 Bread Hugger's. This has a 80% chance of success.

Psychic Projection (Costs 3 Actions). Each creature within 60 feet of the Breadhemoth must make a DC 19 Wisdom saving throw, taking 26 (4d10 + 4) psychic damage on a failed save, or half as much damage on a successful one.

Detect. The Breadhemoth makes a Wisdom (Perception) check.