NightKosh/Gravestone-mod-Graves#52

Add new mob - skeleton with iron sword, burning eyes and sinister, green flame in its chest. It will be spawned in catacombs "tresury rooms", and in some cases at player death in catacombs. This mob will be looks like a pile of bone which will start to assemble in a skeleton when you come close enough to it. Also it will be almost immortal - it will have a chance to escape death, it will fall as a pile of bones instead of it, and reassemble again after some time. This mob will not despawn even on a peacefull gamemode(in this case it will breaks on bones)