The Sage of the Endless Sword, or the Sword Sage, was Yerin's master and the person who taught her Path. He very nearly married Min Sheui, the Winter Sage, who revealed his true name -Timaias Adama- in a portion of her inner monologue that took place in Uncrowned.
The Sage died in the Ancestor's Tomb at Heaven's Glory, poisoned by a bunch of Jades, attacked in his sleep and then killed. There are mysterious, unexplained circumstances surrounding his death, namely that a Jade should not even have been able to hurt the Sage, with poison or madra.
It is later revealed that The Sword Sage was rendered vulnerable thanks to a massive hunger script concealed beneath Sacred Valley. This script consistently saps power from any who enter the valley until their power level is reduced to that of a Jade. Adama, having grown complacent from years of wielding the authority of a sage, underestimated the capacity of this script to weaken him.
In a flashback in Wintersteel, Adama admits to Yerin, he is willingly ingesting poison provided to him by the Heaven's Glory school, believing anything they could make would have no effect on a Sage. He regularly ventures into the Labyrinth via the Ancestor's Tomb and remarks that his trips have been taking more of a toll on him as time has worn on. It is unknown whether this is a result of the poison, or just a side effect of the hunger script beneath Sacred Valley. One night, a dozen elders of the Heaven's Glory school sneak up on Adama in his sleep. They all simultaneously stab him with poisoned half-silver blades. Adama fights back with as much strength as he can muster, eventually making his way into the Ancestor's Tomb where he succumbs to his wounds.
-Wintersteel: His perspective before his death is the prologue