Crystallite 2014

CR 4 giant

Large giant, lawful neutral

Armor Class: 15 (natural armor)
Hit Points: 90 (12d10+24)
Speed: 40 ft.

STR18 (+4)

DEX8 (-1)

CON14 (+2)

INT17 (+3)

WIS11 (+0)

CHA9 (-1)

Saving Throws
Skills: Perception +0
Resistances: cold, fire
Damage Immunities: poison
Condition Immunities: poisoned
Senses: darkvision 60 ft., tremorsense 60 ft., passive Perception 12
Languages: Giant
Challenge: 4 (1,100 XP) · Proficiency Bonus +2

False Appearance.

While motionless indistinguishable from large geode.

Final Form.

When it dies its corpse transforms into crystal and becomes a Large object that can be attacked and destroyed (AC 17; hp 45; immunity to poison and psychic). It can no longer be affected by spells and effects that target creatures.

Reactions

Calcify.

When it takes 5+ damage on a single turn it can reduce its hp max by an amount equal to damage taken and gains a +1 bonus to AC. Reduction and bonus last until it finishes a long rest. Can't increase its AC above 20 using this.

Source: Tome of Beasts 3 (Kobold Press) · License: OPEN GAME LICENSE Version 1.0a details