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.
The data is currently on the state of the New Years 2019 units and Legendary Azura. They will be updated at a later date.

There is a data manager in the works over at the ARDef-Builder-DataManager repository, which will be available to manage the underlying data using a comfortable interface.