The prize of the desolate ruins the Ancestors Spear is sought by all factions of the desolate wilds during the later half of Soulsmith except perhaps Eithan. The Ancestors Spear has the special ability to consume the madra of Sacred artists stabbed by the weapon, funneling their madra into the core of the wielder. While not a perfect weapon it remains an incredibly powerful one like all weapons found in the labyrinth. Also like all weapons found in the Labyrinth it it based upon hunger madra, the study of which is the apparent purpose of the whole Labyrinth.
The spear is first mentioned in Soulsmith, it is known to be the prize of the desolate ruins, an enormous pyramid that rose from the ground a few short days before our protagonists reach it. Upon reaching the ruins they discover the major factions that are hunting within the ruins, the Sandvipers, the Jai clan, and the Fishers, all of whom are competing to get the spear. After several different confrontations and a training montage the spear is found by Yerin and Lindon lead by their newly minted mentor Eithan. However, the Sandvipers find them while they are looting the ruins and in the confrontation the Sandviper heir Kral is killed by Lindon. Before Jai Long can take his revenge Eithan intercedes, saving Lindon but promising Jai Long a campaign of vengance in a year and giving Jai Long the spear.
Over the course of that year Jai Long uses that spear to wage a rebellion against his clan that has since abandoned him. The spear helps him speed his advancement but eventually when confronted by the Jai patriarch Jai Daishou he gives up his rebellion and pledges loyalty in exchange for his sisters life.
The next time we see the spear is during his duel with Lindon. For the first half of the fight he uses a regular spear before pulling the Ancestors Spear from his soul space. Jai Long tends to dominate the fight but Lindon is actually able to destroy the spear during the battle. None the less, Lindon loses his arm during the fight and needs to craft a replacement prosthetic and he actually uses the binding found in the Ancestors Spear. This is what makes his arm the hunger madra weapon that it is now.