Body cores are the corrupted cores of Body Golems, imbued with the power of the Body 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 75 Runecraft experience and ten body runes.
Like regular essence, the amount of body runes crafted will increase with the player's Runecraft level; for example, each body core will craft 30 body runes at level 92 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