As per Vanilla NWN, the DC of the "Imbue Arrow" Fireball Arrow is 10 + AA level + dexterity modifier. After testing it several times on PotM, it seems to be a DC of 10 + AA level only, making the "Imbue Spell" feature /much/ less useful of a tool in an AA's arsenal. Unsure if this is as intended or an unforseen bug.