A soon-to-be online theorycrafting tool for Fire Emblem: Heroes. It can be used to simulate Aether Raid defenses with any unit and any structure.
The FE:H data of the heroes and skills are based on the data used by feh-stuff (already offering a Summon Simulator and Unit Builder on their site, check it out). They have been reorganized to fit the Aether Raid Defense Builder.