Arno can be found in the Guildhall. When Roah Moonglow, Boss Friol, or Trinoda attempts to take over the Guildhall, Arno will join the fight alongside his Zhentarim allies against the Guild. Siding against the Zhentarim will end in Arno's demise, or in his disappearance if he's knocked out.
Deal extra damage to a foe you have Advantage against. Also works if you have an ally within 1.5 m / 5 ft of the target and you don't have Disadvantage.