Uriel was a Time Lord who secretly wanted to take over Gallifrey.
He and Luther started the Final Chapter, a conspiracy to end Rassilon's policy of non-interference and turn Gallifrey into a military power. They were going to create clones from Uriel's biodata as their army. However, when Uriel saw his "first child", Xanti, his military ambitions faded and he put his son in the Chapterhouse, then went into hiding. (COMIC: The Final Chapter)