A sixth level Wizard
spell from the game world AD&D
Invisible Stalker summons an ethereal creature to control as you wish. It is, unsurprisingly, invisible, meaning it cannot be targetted for spells, however it may be affected by area of effect spells. Its invisibility makes it an invaluable ally for small adventuring parties, allowing it to (if allowed) sneak up behind the people who are attacking you and backstab them for triple damage. It also has a moderate resistance to cold and poison, and being magical, has VERY low saving throws for all but magic.
Over the various editions of AD&D, the abilities have changed somewhat for the Invisible Stalker. Current forms have it at 8 hit dice. Equipped with claws for hands, it uses these as weapons, counting as magical strikes, causing 2d4+2 damage, and it has 4/2 attacks (i.e. 4 attacks every 2 rounds).
Also makes an appearance in Baldur's Gate, I and II, and is a very useful spell to research if you can afford it.