Edimmu are the result of a spirit failing to leave a dead body and are found in a large chamber through the barrier on the northern side of the Motherlode Maw Resource Dungeon in Prifddinas.
They have the ability to heal 25% of the damage they deal, which combined with their already very high life points, can make them quite slow to kill. Therefore it may be beneficial to use Protection Prayer/Deflect curses to reduce their damage and therefore the amount they can heal per hit.
Despite having a Slash weakness, using Magic or Ranged seems to be a much more effective way to kill Edimmu and has the added bonus of negating their Melee attack if you fight them from a distance. However, as they are able to use Magic from a distance themselves, using Ranged may be preferred over Magic.
There is a 1/50 chance that you may spawn a stronger level 125 Edimmu after killing a normal one. This stronger version has a chance to drop a pet Edimmu called Eddy when killed. Note that wearing a Ring of wealth will slightly increase the chance of spawning a stronger Edimmu to 1/47.
Edimmu can also be encountered while Dungeoneering and are the only monster which drop a Blood necklace.
The chance of receiving better drops is increased by the Ring of fortune, Enhanced luck potion, Collector's insignia (charged), or Luck of the dwarves.