Temüjin began his ascent to power by offering himself as an ally (or, according to other sources, a vassal) to his father's anda (sworn brother or blood brother) Toghrul, who was Khan of the Keraites, and is better known by the Chinese title "Wang Khan", which the Jurchen Jin dynasty granted him in 1197.
What tribe did Toghrul lead?
the Keraites