Crystal Elemental (Minor)

Earth Glide. The elemental can burrow through nonmagical, unworked earth and stone. While doing so, the elemental doesn't disturb the material it moves through.

Harmony. Whenever the elemental takes thunder damage roll a d6. On a result of 1-3 the resonance of the elemental cancels out all the damage, on a result of 4-6 the resonance amplifies its effect and the thunder damage is doubled. 

Multiattack. The elemental makes two slice attacks or uses it's resonant shriek .

Slice. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) piercing damage.

Resonant Shriek (Recharge 6).  A soft hum begins from the core of the elemental and builds to painful pitch. All other creatures within 30ft. that can hear the noise must make a DC-15 Con save or suffer 10 (4d4) points of thunder damage. If a creature fails the save by more than 10 they are additionally incapacitated until the end of their next turn. 

Armoured Form. Reaction: The first time the elemental is brought below half health it may use its reaction to grow a protective crystal shield. The elemental gains 5 (2d4) temporary hit points and gains resistance to all damage except psychic and thunder. This reaction can only be taken if the elemental is in contact with natural ground (earth, rock, crystal, etc). If the elemental is in a crystal elemental lair it gains an additional 5 (2d4) temporary hit points for a total of 10 (4d4).