Dead Hand is an undead monster that appears as a mini-boss in The Legend of Zelda: Ocarina of Time's well in Kakariko Village and Shadow Temple. It's unknown if he served anyone, but if this is the case or not Link will still be required to defeat it in order to get the Lens of Truth and Hover Boots depending on where you battle it. It should also be noted that in the Master Quest version of the game, there are even more Dead Hands you'll have to beat.
The Dead Hand is quite an odd and terrifying looking creature. His jaw opens extremely wide, and he'll use it in order to try and engulf the protagonist, though his tiny arms won't frighten the player too much. Because of its appearance, many consider it one of the most frightful enemies in the entire series.