The dark energy core is a monster that is first seen in the Summer's End quest. It appears from within the Spirit Beast and players have to trap the Core within a hole, in order for Jallek Lenkin to attack the Beast. According to Jallek, the dark core contains a portion of the player's spirit within it, as the beast has been draining it during Spirit of Summer and Summer's End. After the player and Meranek Thanatos break the chestplate protecting the dark core, it will fall out and, as it contains sections of the players spirit within it, will try to rejoin with the player's spirit. However, as it had been corrupted with dark energy, trying to unite with the player's spirit would cause much pain.
The dark energy core also appears from within the Corporeal Beast. When the core appears is dependent on how many players are in the room and how fast the Beast is losing health. Although the core has combat stats, it does not attack with a combat style; instead it deals classless damage onto anyone within a 3x3 radius of it. Players hit by the core will lose health, which the core transfers to the Beast to regain lost health and drain some prayer points every hit. The low amount of health the core has means that it can easily be killed through abilities.
Rarely, after killing the Corporeal Beast, the Dark core may still be running around as usual. It will still stay in the same spot, damaging players until they move. The only way to fix this is by staying away from the 3x3 radius and after jumping to the next spot, it will disappear.