To reach this rank you'll need to complete two quests; "Retaking the Fringe" and "Rebuilding the Gatekeeper". In "Retaking the Fringe", you'll need to travel to The Fringe and speak with the commander. Knights of Order will appear, defeat them and enter Xeddefan. You'll need to locate the obelisk and deactivate it. Be careful though, as there are many enemies in this area. When the obelisk has been deactivated, return to Sheogorath.
You'll now start "Rebuilding the Gatekeeper". Make your way to Xaselm and talk to Relmyna Verenim. She will tell to collect body parts from the Gardens of Fresh and Bone . Once you have the items, return to Relmyna and she will bring the new Gatekeeper to life. Return to Sheogorath and you'll become a Regent.
Refer to Retaking the Fringe
and Rebuilding the Gatekeeper
for more information.