Crystal Shard Elemental

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

Reverbertating Crystal: Whenever the Crystal Shard Elemental is affected by a sound based effect the effect is recast centered on it with in all directions with its range halved and only once per attack or effect.

The Crystal Shard Elemental is not immune to these effects however.

Magic Resistance: The Crystal Shard Elemental has advantage on saving throws to avoid spells and spell like effects.

Magic Reflection: Whenever the Crystal Shard Elemental is affected by a spell of 6th level or lower the caster must roll a d6, on a 4 or 5 the spell fails to affect the Crystal Shard Elemental on a 6 the spell is reflected towards the caster, being recast targeting it or with it as the center of the spell area if possible.

Radiance Surge: Whenever the Crystal Shard Elemental would take radiant damage it instead reflects that damage (this effect hapens before damage calculation ignoring effects that would ignore resistance or immunity to radiant damage), all creatures within 15ft of the Crystal Shard Elemental must succeed in a DC 12 Constitution saving throw taking radiant damage equal to half of the damage that would have been dealt to the Crystal Shard Elemental instead.

Immutable Form: The Crystal Shard Elemental is immune to effects that would alter its form such as the polymorph spell.

Crystal Shrapnell: Whenever the Crystal Shard Elemental falls to 0 hit points it can choose to explode into shards of crystal, if it does so all creatures within 10ft of it must succeed in a DC 12 Dexterity saving throw taking 1d6 slashing damage on a failure.

Crystaline Slam: Melee Weapon Attack +4 to hit, 5ft reach, one target, for 1d6+2 slashing damage.

Crystal Bullet: The Crystal Shard Elemental throws itself towards a creature within 30ft of it, the target must succeed in a DC 12 Dexterity saving throw taking 3d6+2 slashing damage on a failure.