The final ingredient
Once the Great Welkynd Stone is delivered, the Champion is shocked to discover the last item for Martin's ritual. Martin will require the Great Sigil stone - only found in the Great Oblivion Gate. They will have to let the Mythic Dawn open the Great Gate despite the risk to Martin's safety.
The Great Gate
The planes of Oblivion within the gate is a single level map with a number of scamps, both fire wielding and melee attacking. The Great Sigil Stone is in the most North-western corner of the map. Once the Stone is activated, the Champion will take it. It is then merely a matter of returning to the Oblivion gate at the level's entrance. Once the gate is activated, the quest will end.
