Mirror Spawn (5e Creature)
From D&D Wiki
Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d6 + 6) slashing damage.
Diamond Slice. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 21 (3d10 + 5) slashing damage.
Shatter spark. As a reaction to being targeted by a spell that deals lightning damage, the spell will ricochet and turn into the shatter spell targeting a point of the mirror spawn's choosing within the spells range.
Duplicate. When a creature attacks or cast a spell on a mirror spawn, the spawn can create 3 duplicates of itself in unoccupied spaces it can see with 20 feet of itself. These duplicates have 10 hit points and do not attack, but at the start of end of a mirror spawn's turn, it can swap places with a duplicate.
Burning Glass. As a reaction to taking fire damage, a mirror spawn can absorb the heat causing its next attack to deal 1d8 additional fire damage until the end of its next turn.
These humanoid glass creatures are native to the mirror plane. There are very rare windows into the mirror realm scattered deep in diamond mines. When a person looks into the mirror realm windows it will appear as if they are looking into a mirror. Any attempt to touch the mirrors will have your mortal body fall unconscious while your soul is taken into the mirror realm. The only way to escape is by destroying your mirror spawn. The mirror spawn are territorial creatures and do not enjoy others mining away around their windows.