Ether Elemental (5e Creature)
From D&D Wiki
Large elemental, neutral
Damage Resistances bludgeoning, piercing, and slashing from nonmagical attacks
Ethereal Form. The elemental can move through a space as narrow as 1 inch wide without squeezing. A creature that touches the elemental or hits it with a melee attack while within 5 feet of it takes 7 (2d6) force damage. In addition, the elemental can enter a hostile creature's space and stop there. The first time it enters a creature's space on a turn, that creature takes 7 (2d6) force damage.
Slam. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 17 (2d12 + 4) force damage.
Among the four elemental planes is a theoretical fifth, ether. Most scholars heavily debate the existence of such a thing as it would completely change all they knew of the elements. One of the most pressing arguments for the existence of this fifth element is the elusive ether elemental. A fuzzy humanoid shape made of violent prismatic energy, roaming the ethereal planes. Ether elementals are commonly drawn out by attempts to travel through different planes of reality.