An Arena Medal is an expendable item from Fire Emblem Heroes
Arena Medals are used in the Weapon Refinery mechanic and was introduced in the Version 2.0 update for Heroes. Arena Medals are used in all methods of the Weapon Refinery feature.
Regardless if the weapon being refined is a common or Legendary weapon, the costs to refine for skills or upgrade weapons costs the same for all weapons. Refining a weapon to gain a new passive skills costs 500 Arena Medals. Upgrading a weapon, such as Assassin's Bow+ into the Guard Bow+ or Durandal into Blazing Durandal costs 200 Arena Medals.