When killed, some release a gas that can damage for 20-30 life points over time. 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. The skeletal undead ones do not release this gas.
They can be used as a substitution for Zombies or Skeletons as a Slayer task, depending on which ones are killed, for higher levelled players who wish to get more experience from their task. 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.