Stalfos are a race of skeletal people. Some say that when a Hylian becomes lost in the Lost Woods that they become a Stalfos, a member of the undead. They have a child version called the Stalchild which may be children Hylians that have become lost. They also appear to share a relation to other undead such as Stalkin, Stalhounds, Staltroops, and are apparently ruled by the Stallord. Some varients include the Stalfos Knight, Stalfos Reaper, Master Stalfos, and the Grim Creeper.
Stalfos are often shown to be adept fighters that use both swords and hammers to attack others and they can use a shield for a defense. This is a skill which becomes their signature in Legend of Zelda: Online they are very good at using hammers and shields at the same time, possibly better than any other race.
In some forms however there are two types of Stalfos the Red Stalfos are the ones described above that use the weaponry and their skills and the Blue Stalfos can throw bones at people from a distance, this is a trait that they can also use in Legend of Zelda: Online.
Stalfos have the uncanny ability to revive themselves in many games. If they are defeated they will turn into a pile on the ground much like Dry Bones from the Mario series, however over time they will revive themselves. They only way to defeat them for good is to put a bomb on their bones and let it explode and shatter their bones for good.
A small group of Stalfos make an appearance in this fanfic, assaulting captain Vegas' group as they were hunting down the sole survivor of the village of Sorac. Unintentionally saving the child, their ambush proved ineffective as they were quickly beaten into the ground by the trained soldiers and their skillful use of maces. It should be noticed that if it had not been for the Stalfos, the warriors would have most likely never encountered Link.
Based on this encounter, it appears that bludgeoning weapons such as maces are extremely effective when it comes to fighting these skeletal undead, seeing as they are capable of breaking their bones beyond repair.
A wide variety of Stalfos appear in Lunatic Entertainment's primary Zelda game, Dark Legacy. Depending upon the choices made by the player, the Stalfos enemy type can become one of the most common types in the game in terms of sheer variety and number. Should Dark Link choose to side with Ganondorf again, he can eventually help one of Ganon's minions, the Necromancer, to raise the dead and complete flood Hyrule Field with an army of Stalfos of all types.
At current count, 32 variations of Stalfos -- not including subtypes such as Stalchildren, Stalfos Knights, or Shrouded Stalfos -- can be found in the game.