The Vanari Lord Regent is the highest position attainable by a member of the Vanari caste of the Lumineth Lords. They are the consummate generals and leaders of armies, able to veto and launch invasions at any time. Lord Regents - as the name suggests - are regents of Tyrion's will, versed in the Tyrion Decree and speaking with his voice and authority in the debates and councils of the Teclian nations. You really don't want to get caught up in such debates, though, as the only places where the Lord Regent feels truly at home are in the training grounds, the strategists' halls, and the white-hot crucibles of battle.
In order to be made Regent Lord, one must undergo several trials of worthiness devised by Tyrion himself - these include traveling to the edge of Haixiah and returning illuminated, as well as taming her Lightcourser mounts.