Mob: Umbra

A new Mob called an Umbra; which looks like a shadow with eyes that can go through all blocks except cobblestone, any glass, and obsidian. it is neutral unless the player is wearing armor not made of leather, or holding a sword; which then it will become hostile and switch between a Stone spear, stone sword, or a bow depending on distance. Spawns in darkness though not necessarily at night. has ~40 HP. it de-spawns in daylight. finds saplings and makes them grow.
Drops:

• Sticks
• Coal
• Stone spear (rarely)
• Umbra dust which you can craft into bonemeal, or an Umbra helmet that increases your jump height, scares all mobs, and nourishes crops

It would spawn in forest biomes.