Original:  Any scion of an eligible heir that did not live long enough to ascend to the throne was cast aside as not eligible, creating a pool of discontented pretenders called Tegin in Turkic and Izgoi in Rus dynastic lines.
Revised:  Any scion of an eligible heir who did not live long enough to ascend to the throne was cast aside as not eligible, creating a pool of discontented pretenders called Tegin in Turkic and Izgoi in Rus dynastic lines.
Edit intent: fluency