Possible explanation for Mantid's backstory:If we go on the assumption that the Brothers' weapons of choice have to do with their previous life, then Mantid was definitely a spear-wielder — probably a disposable, light infantry soldier, since those were the people who most commonly used spears (though he could also have been a master fighter or a forger of spears, since he has so many of them). He was probably killed by a spear strike as well, as evidenced by the spear through his torso. However, it's interesting to note that the spear is oriented as if he was stabbed In the Back. Perhaps this indicates that he was killed through a betrayal?
The Scorpio are the denizens of a Limit that has fallen into the NightmareThe genrally rotten look of the Gnats implies they died. And The Brothers rose from the Nightmare, and became the victims of much Body Horror because of it. Therefore, Everyone in The Void, except the one who asceneds, becomes an entity like the Scorpio.