Sage of the Endless Sword

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 (Adama) in a portion of her inner monologue that took place in Uncrowned.

Death

The Sage died in the Ancestor's Tomb at Heaven's Glory, poisoned by a bunch of Jades 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.

There is much speculation surrounding this fact, and fans constantly theorize about it on the Cradle subreddit (r/Iteration110Cradle), so much that it has become a fandom joke.