Bysmuth

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

Aversion to Thunder. If Bysmuth takes thunder damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.

Discharge. Bysmuth's successful attacks discharge any charge stacks on the target dealing an additional 1d6 lightning damage per stack.

Legendary Resistance (3/Day). If Bysmuth fails a saving throw, it can choose to succeed instead.

Unusual Nature. Bysmuth doesn’t require sleep.

Multiattack. Bysmuth makes two Static Rend attacks, and it uses Synaptic Shock.

Static Rend. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage plus 7 (2d6) lightning damage.

Synaptic Shock. Bysmuth targets one creature it can see within 5 feet of it. The target must succeed on a DC 15 Wisdom saving throw or take 9 (2d8) psychic damage and 5 (2d4) lightning damage. Additionally the target is afflicted by Synaptic Static until it finishes a short or long rest or until it benefits from the greater restoration or heal spell. Constructs, Oozes, Plants, and Undead succeed on the save automatically.

While affected by the static, the target must roll a d4 and subtract the number rolled from its ability checks and attack rolls. Each time the target is memory drained beyond the first, the die size increases by one: the d4 becomes a d6, the d6 becomes a d8, and so on until the die becomes a d20, at which point the target becomes unconscious for 1 hour. The effect then ends.

Any creature that fails the saving throw gains one stack of charge.

Synaptic Storm (Recharge 5-6). Bysmuth uses Synaptic Shock on every creature within his influence.

Spellcasting (Psionics). Bysmuth casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 15):

3/day each: charm person (as 5th-level spell), detect thoughts, hypnotic pattern

Crystalline impersonation. Bysmuth extrudes a piece of itself that assumes the appearance of one Medium or smaller creature whose memories it has stolen. This simulacrum appears, feels, and sounds exactly like the creature it impersonates, though it smells faintly of sulfur. Bysmuth can impersonate 1d4 + 1 different creatures, each one tethered to its body by a crystal matric that can extend up to 120 feet away. The simulacrum is an extension of Bysmuth, meaning that Bysmuth occupies its space and the simulacrum’s space simultaneously. The tether is immune to damage, but it is severed if there is no opening at least 1 inch wide between Bysmuth and the simulacrum. The simulacrum disappears if the tether is severed.

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

Stolen Visage. Bysmuth generates a simulacrum at a chosen location within his influence and makes a Static Rend attack from its location.

One With the Shards. Bysmuth sinks out of sight into the ground. On his turn he reappears on any unoccupied space within his influence.

Synaptic Storm (Costs 2 Actions). Bysmuth uses Synaptic Storm if available.