Eye of the Nebula
Enderman, Enderman! Does whatever the Ender can...
It can be found in chests generated within End Cities and, much more rarely, Strongholds.
While equipped in Spellstone slot, it will give the bearer 65% Magic Damage Resistance and 15% Chance to teleport away from any attack without receiving any damage. When triggered, it's active ability will teleport you right behind the creature you are currently looking at.
Spellstone's active ability ignores any obstacles when searching for entities. Also, it doesn't check location safety before teleporting the user - so, the position of player could be in blocks, liquids or in the air. But it will always be on the same Y coordinate as a targeted entity and about one block behind it.
After teleporting the player behind an entity, it sets them to be looking right at the targeted entity, allowing to instantly attack it. With quick enough reaction, you can even deal a critical hit. The only exception could be exceptionally small or unusual entities, like fishes - then player's crosshair usually ends up a bit above the target.
When teleporting away from attack, spellstone searches for a (relatively) safe location within 16 blocks radius from them. It will always be a solid block with two blocks of air above it. If there are any caves or hollow spaces within this radius, you might occasionally get into them, although usually it's not very likely. This ability might not trigger if there aren't any valid locations within this radius, or there are very few.