Crystal Elemental

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

Reverbertating Crystal: Whenever the Crystal 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 Elemental is not immune to these effects however.

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

Magic Reflection: Whenever the Crystal 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 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 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 Elemental must succeed in a DC 16 Constitution saving throw taking radiant damage equal to half of the damage that would have been dealt to the Crystal Elemental instead.

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

Siege Monster: The Crystal Elemental deals double damage to structures.

Multiattack: The Crystal Elemental makes two melee weapon attack on its attack action or one weapon attack and one of its available actions.

Crystal Slam: Melee Weapon Attack +8 to hit, 5ft reach, one target, for 2d8+5 bludgeoning + 1d8 slashing damage.

Reflection Beam (Recharge-6 or otherwise if it would take radiant damage): All creatures within a 60ft long 5ft wide line centered on the Crystal Elemental must succeed in a DC 16 Dexterity saving throw taking 4d6+5 radiant damage on a failure.