Axiom - The Shadow of Wregate

Amorphous. Axiom can move through a space as narrow as 1 inch wide without squeezing.

Corrosive Form. A creature that touches Axiom or hits it with a melee attack while within 10 feet of it takes 10 (3d6) acid damage. Any nonmagical weapon made of metal or wood that hits Axiom 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.

Nonmagical ammunition made of metal or wood that hits the pudding is destroyed after dealing damage.

The pudding can eat through 2-inch-thick, nonmagical wood or metal in 1 round.

Spider Climb. Axiom can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Magic Resistance. Axiom has advantage on saving throws against spells and other magical effects.

Legendary Resistance. 3/day. If Axiom fails a saving throw, he can choose to succeed instead.

Aged Exterior.  Axiom is an ancient, elder black pudding and has developed a defensive coating on his body making him immune to all forms of damage except for a special concoction which makes him vulnerable. 

Regeneration. Axiom regains 10 hit points at the start of its turn. If Axiom takes fire damage, this trait doesn't function at the start of his next turn. Axiom dies only if his starts his turn with 0 hit points and doesn't regenerate.

Innate Spellcasting. Axiom's innate spellcasting ability is Charisma (spell save DC 22, +14 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: charm person,detect magic, detect thoughts, suggestion

3/day each: hold person, Clairvoyance, dispel magic, 

1/day each: banishment

Spellcasting. Axiom is a 10th-level spellcaster. His spellcasting ability is Charisma (spell save DC 22, +14 to hit with spell attacks). It requires no material components to cast its spells. Axiom has the following sorcerer spells:

Cantrips (6 slots): acid splash, control flames, infestation, mage hand, mold earth, shocking grasp

2nd level (5 slots): darkness, enlarge/reduce, snilloc's snowball swarm

3rd level (5 slots): counterspell, fly, haste

4th level (4 slots): blight, greater invisibility, vitriolic sphere

5th level (3 slots): cone of cold, dominate person, synaptic static

Metamagic - Empowered Spell. When Axiom rolls damage for a spell, he can spend 1 sorcery point to reroll up to 5 of the dice. He must use the new rolls and he can use this option even if he has used another Metamagic option.

Metamagic - Quickened Spell. When Axiom casts a spell that has a casting time of 1 action, he can spend 2 sorcery points to change the casting time to 1 bonus action for this casting.

Metamagic - Twinned Spell. Axiom can spend sorcery points equal to a spell's level (1 for cantrip) to target a second creature in range with the same spell, as long as the spell at the level it's being cast is incapable of targeting more than one creature.

Strength of the Grave. When damage (that isn't radiant) reduces him to 0 hp, he can make a CHA saving throw (DC 5 + the damage taken). On a success, he instead drops to 1 hp (and can't use this feature again until he finishes a long rest). 

Multiattack. Axiom makes 2 Pseudopod attacks.

Pseudopod. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 17 (4d6 + 5) bludgeoning damage plus 18 (4d8) acid damage. In addition, nonmagical armor worn by the target is partly dissolved and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10.

Sorcery Points: Axiom has 10 sorcery points that he regains when he finishes a long rest.

Convert Sorcery Points: Axiom can use his Sorcery Points to gain additional spell slots or sacrifice spell slots to gain additional sorcery points as a bonus action.

Shadow Walk: When you are in dim light or darkness, as a bonus action, you can magically teleport up to 120ft. to an unoccupied space you can see that is also in dim light or darkness.

Split. When a pudding that is Medium or larger is subjected to lightning or slashing damage, it splits into two new puddings if it has at least 10 hit points. Each new pudding has hit points equal to half the original pudding's, rounded down. New puddings are one size smaller than the original pudding

Spells. Counterspell (3rd)

Axiom 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. Axiom regains spent legendary actions at the start of its turn.

Pseudopod. Axiom makes a Pseudopod attack.

Sleep Ray. The targeted creature must succeed on a DC 22 Wisdom saving throw or fall asleep and remain unconscious for 1 minute. The target awakens if it takes damage or another creature takes an action to wake it. This ray has no effect on constructs and undead.

Petrification Ray(Cost 2 Actions). The targeted creature must make a DC 22 Dexterity saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.

If Axiom's health falls below 30 HP, he can use the option below as a legendary action.

Bug Out (Costs 3 Actions). Axiom casts Shocking Grasp on himself, forcing himself to split into two and then proceeds to Shadow Walk as a bonus action away from the battle. He will typically choose a tiny crevice near his lair where he will hide and proceed to take a long rest.