The Undead ones play a critical part in the Shilo Village quest. They are found in Rashiliyia's Tomb, outside of Shilo Village. This makes them decent training for people training Range, Magic, and meleers wielding halberds.
When killed, some release a gas that can damage 20-30 life points. However it is not poisonous, so antipoison is not required. The gas only affects characters in a 1-square radius around where the undead one was when it was killed, so if players kill one they have a few seconds to run to avoid taking damage.
They can be used as a substitution for Zombies as a Slayer task for higher levelled players who wish to get more experience from their task, or those who use Duradel/Lapalok. They do not drop anything, though, and so are rarely killed.
Players can earn around 37,000-40,000 combat experience per hour fighting undead ones.