Its either the husk of the pit, eidolon ally or necrochasm Edit : first of all, this is a fantastic riddle that really gets you thinking, props to the makers. It ceases to exist because the blades of crota have been taken out of the game, making the husk unobtainable. It has or can evolve due to the husk being able to upgrade into the eidolon ally and necrochasm afterwards. People scavenge for it because you need to hunt several blades to get it. When you shine your light on it you are rewarded with the eidolon ally. Three in one, one in three, refers to husk, eidolon ally and necrochasm. Everything in the riddle points toward the husk of the pit. If I am wrong I will dismantle all my exotics