As the Grand Archmage of the second tower, he has wrongfully banned Ariane from the Wizards Tower for demon-worshipping, as shown in the Lore "The Burning Tower" and dialogue with him during Rune Mysteries, even going so far as to cast a ward to prevent her entry.
During the events of Rune Mysteries, he refuses to admit Ariane into the tower, reassuring that he has prepared a ritual to cast Icyenic Purge in order to rid the tower beam of the vortice that attacked Wizard Borann. During the ritual, however, he is distracted by the player playing the lower manuals of the organ in order to buy time for Ariane to cast the proper spell and save the day.
After the quest Rune Memories, Sedridor will be saddened by the betrayal of Wizard Ellaron and offer players to take from his personal rune essence chest after the quest which yields essence equal to a base amount + your runecrafting level*5. He will also offer the Archmage title to players who have 99 Magic and Runecrafting.
Sedridor used to be the head wizard and chief of research at the Wizards' Tower. In the old Rune Mysteries quest, Sedridor, having struggled to rediscover the knowledge of runestone creation, which was lost after the destruction of the Tower by Zamorakian mages decades ago, recently succeeded in this task largely due to the efforts of an adventurer who brought him an unidentified air talisman from Lumbridge's Duke Horacio, who knows of Sedridor and his vast lore well enough to send it to him for examination. He is located in the tower's basement. You are required to talk to him twice during the Rune Mysteriesquest. After the said quest, you can use him to teleport to the rune essence mine, where you can mine rune or pure essence. He is seldom visited for this, because he is so far away from a bank. Sedridor has conducted extensive research on runes and the secrets of crafting runestones, eventually combining his research with that of Aubury's rune knowledge to unlock the secrets of the Air talisman, thus considerably increasing the Wizards' supply of runes and leading to the opening of a Runecrafting Guild, which is run by a friend of Sedridor's, Wizard Elriss.
Archmage Sedridor is one of the four NPCs who can teleport to the sandcastle building area in the Summer Beach Party. While he is present, building the Wizards' Sandtower (a replica of the Wizards' Tower) will grant double experience.
Before the graphical update of the wizards tower, Sedridor could be found in the tower basement.
Archmage Sedridor will not allow players to bank essence at the deposit box, which can be found at the very top floor of the Wizards' Tower. This may be a conscious decision by Jagex to prevent an easier way of power-mining rune/pure essence. Upon attempting to do so, a message will appear stating: "Sedridor does not want you banking rune essence in the tower."