As I understand it, there is no guild of mages in the first campaign, so who should Arantir fight with? Only as an avatar? By the way, considering that Necropolis has little mana, you can summon an avatar and replenish mana gradually using spiritual connection.
There are guilds, but the new spells are locked. You can summon and replenish, but then the avatar will die, and ALL the mana will disappear... So...