Death Slaad

Death Slaad CR: 10

Medium aberration (shapechanger), chaotic evil

Armor Class: 18

Hit Points: 170 (20d8+80) 20d8+80

Speed: , fly: 0 ft

STR

20 +5

DEX

15 +2

CON

19 +4

INT

15 +2

WIS

10 +0

CHA

16 +3

Saving Throws: spells and other magical effects.

Skills: Arcana +6, Perception +8

Damage Resistances: acid, cold, fire, lightning, thunder

Senses: blindsight 60 ft., darkvision 60 ft., passive Perception 18

Languages: Slaad, Telepathy 60ft

Challenge Rating: 10

1/day: cloudkill, plane shift

2/day: fear, fireball, fly, tongues

Shapechanger.
The slaad can use its action to polymorph into a Small or Medium humanoid, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
Innate Spellcasting.
The slaad's innate spellcasting ability is Charisma (spell save DC 15, +7 to hit with spell attacks). The slaad can innately cast the following spells, requiring no material components:
At will: detect magic, detect thoughts, invisibility (self only), mage hand, major image 2/day each: fear, fireball, fly, tongues 1/day each: cloudkill, plane shift
Magic Resistance.
The slaad has advantage on saving throws against spells and other magical effects. Magic Weapons.
The slaad's weapon attacks are magical. Regeneration.
The slaad regains 10 hit points at the start of its turn if it has at least 1 hit point.

Actions

Multiattack.
The slaad makes three attacks: one with its bite and two with its claws or greatsword. Bite (Slaad Form Only).

Melee Weapon Attack:
+9 to hit, reach 5 ft., one target. Hit: 9 (1d8 + 5) [roll1d8+5] piercing damage plus 7 (2d6) 2d6 necrotic damage. Claws (Slaad Form Only).

Melee Weapon Attack:
+9 to hit, reach 5 ft., one target. Hit: 10 (1d10 + 5) [roll1d10+5] slashing damage plus 7 (2d6)  2d6 necrotic damage.

Greatsword.Melee Weapon Attack:

+9 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) [roll2d6+5] slashing damage plus 7 (2d6)  2d6 necrotic damage.
Variant: Control Gem
Implanted in the slaad's brain is a magic control gem. The slaad must obey whoever possesses the gem and is immune to being charmed while so controlled.
Certain spells can be used to acquire the gem. If the slaad fails its saving throw against imprisonment, the spell can transfer the gem to the spellcaster's open hand, instead of imprisoning the slaad. A wish spell, if cast in the slaad's presence, can be worded to acquire the gem.
A greater restoration spell cast on the slaad destroys the gem without harming the slaad.
Someone who is proficient in Wisdom (Medicine) can remove the gem from an incapacitated slaad. Each try requires 1 minute of uninterrupted work and a successful DC 20 Wisdom (Medicine) check. Each failed attempt deals 22 (4d10)  2d6 psychic damage to the slaad.