In the same dungeon as you find the Titan helmet just follow the linear path of the dungeon. The Titan scepter is located in a chest just up some staits from the Titan helmet room guarded by some undead creatures (locked chest difficulty 3). The dungeon is to the east of the Don's camp. You will need to cross some lava with the Levitation spell to get there.
You can also buy it from Karlsen at the monastery at some point just in case you don't have the lockpick skill for the chest.