These Bones are acquired in the Soul Wars minigame as drops from enemy players. They look just like normal bones but have a different examine text and a destroy option instead of drop, can be used on teammates in Soul Wars to give it to them, give no prayer experience, and can be buried very quickly by clicking on one multiple times until they are all used up.
They can only be buried in a graveyard controlled by the players' team or in the team base. The bones have a slight chance of restoring that team's Avatar's slayer level and restores 50% of your prayer when buried (see Soul Wars) so that some enemies will not have the slayer requirement to kill the avatar anymore.
Players that die in Soul Wars will drop one set of bones plus all of the bones they have and all soul fragments.