Try not to go overboard with trying to sound old. The voice should sound like an older man and should express apparent age but don't sound TOO old ie: like the generic old man character in most anime dubs. 

Character description from the Dark Souls wiki

Gwyn, Lord of Cinder was greatest of the gods and a mighty warrior. He was known as the Lord of Sunlight and headed the efforts to end the Age of Ancients, together with Gravelord Nito, the Witch of Izalith, and the dragon-traitor, Seath the Scaleless. Leading his knights, he led the Gods to victory and oversaw the Age of Fire. Despite bequeathing elements of his soul to the Four Kings and Seath the Scaleless, his soul remains "a powerful thing indeed."

Around a thousand years prior to the start of the game, Gwyn sacrificed himself to link the First Flame. His reasons for doing so are disputed, but what is certain is that his principal motivation was to postpone the end of the Age of Fire. He was succeeded by his children: his firstborn, the banished God of War who lost his status as a deity; his daughter Gwynevere, the Queen of Sunlight; and his lastborn Gwyndolin, his "repulsive [and] frail" son who Gwyn raised as a daughter due to his affinity for the moon.