Compendium

Spellcasting. The Compendium is spellcasting ability is Inteligence (spell save DC 18, +8 to hit with spell attacks). The Compendium has following spells prepared:

At will cast: Chill Touch, Eldritch Blast, Frostbite, Guidance, Mind Sliver, Ray of Frost, Sacred Flame, Shocking Grasp, Sorcerous Burst, Starry Wisp, Toll the Dead, Aganazzar's Scorcher, Arms of Hadar, Bane., Confusion, Elemental Bane, Entangle, Ice Knife, Inflict Wounds, Invisibility, Magic Missile, Mind Spike, Misty Step, Sanctuary, Wither and Bloom,
Leveled slots (12 slots): Blight, Blur, Burning Hands, Call Lightning, Chaos Bolt, Chromatic Orb, Cloud of Daggers, Counterspell, Erupting Earth, Fireball, Fire Shield, Guiding Bolt, Heat Metal, Hunger of Hadar, Ice Storm, Lightning Bolt, Polymorph, Scorching Ray, Shatter, Shield,  Storm Sphere, Tasha's Caustic Brew, Thunderwave, Vitriolic Sphere, Wind Wall,
Smite Magic (8 slots): Blinding Smite, Divine Smite, Searing Smite, Shining Smite, Staggering Smite, Thunderous Smite, Wrathful Smite,

Magic Absorbant. When getting damaged by spells and/or magical effects, the Compendium regains a spell slot back for every 10 damage.

Mutation Control. If a spell like polymorph is used on The Compendium, it can choose the form it changes to as a result of the spell.

Ooze-like Consistency. The Compendium can move through a space as narrow as 1 inch wide without squeezing. Aditionally, any weapon made of metal, stone, leather, and/or wood that hits or comes into contact with the Compendium corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. Ammunition made of metal, stone, leather, and/or wood that hits the Compendium is destroyed after dealing damage. The Compendium can eat through 6-inch-thick, metal/stone/wood in 1 round.

Digest. If the Compendium has consumed an item or an object during the present or past round, it regains 2d8 health.

Aquagenic Urticaria. The Compendium is allergic to water and will take 2d10 upon contact with it.

Great Slam. Melee Weapon Attack: +7 to hit, reach 10 ft. Hit: (3d6 + 4) Magical Bludgeoning damage.

Harden (2 uses) (Recharges after a Short or Long Rest). The compendum reduces its speed by 20ft but raises its AC by 5. This lasts until the end of its next turn. While in this state, the compendium cannot Enlarge or Reduce.

Enlarge (3 uses) (Recharges after a Short or Long Rest). For 1 minute, the Compentium magically increases in size. While enlarged, it is Huge, doubles its damage dice with its Great Slam attack, and makes Strength/Athletics checks and Strength saving throws with advantage. This lasts until the end of its next turn. While in this state, the compendium cannot Harden or Reduce.

Reduce (4 uses) (Recharges after a Short or Long Rest). For 1 minute, the Compentium magically reduces in size. While reduced, it is Medium, increasing its AC by 2, doubles its speed and makes Dexterity/Acrobatics checks and Dexterity saving throws with advantage. This lasts until the end of its next turn. While in this state, the compendium cannot Harden or Enlarge. 

Trip. When the Compendium is attacked by a creature within 5 to 10 feet, it can use its reaction to make them reroll their attack. If the creature rolls a nat 1, they go prone.

The compendium has 2 legendary actions. They recharge at the begining of its turn.

Arcane Offensive (Costs 1). Castas a spell of its choice.

Chargeslam (Costs 1). Moves towards a creature it can see within its reach, uses Great Slam.