Chaos cores are the corrupted cores of Chaos Golems, imbued with the power of the Chaos Altar. They can be used in place of rune or pure essence, and acts as if the player was carrying ten essence instead of one, meaning each core gives 85 Runecraft experience and ten chaos runes.
Like regular essence, the amount of chaos runes crafted will increase with the player's Runecraft level; each chaos core will craft 20 chaos runes at level 74 Runecraft.
Golem cores cannot be placed in a runecrafting pouch. If players attempt to store them in one, a chatbox message will appear, stating "The golem core is too big to fit in an essence pouch."
For an exhaustive list of all known sources for this item, see here