Heroic Grails are an item and a currency in Fire Emblem Heroes.
Heroic Grails are used to summon Grand Heroes or Tempest Trials units, allowing players to acquire extra copies of said units. Heroic Grails are obtained by participating in Aether Raids. In the beginning of the game, they give you 200 Heroic Grails. Players earn a fixed amount each week depending on their ending Rank and Lift Tier.
Each Hero summoned via Heroic Grails costs 100 Grails to summon them the first time. Each subsequent summon requires 50 more grails, capping at a maximum of 500 Grails per summon once a unit has been summoned 8 times.