MASTER8 5e, but many elements of it can be used for other tabletop RPGs, such as D&D 3.5e and Pathfinder RPG. And of course, because everything can be completely customized, any homebrewed content is also possible.
MASTER8 The app is free to download, but you are limited creating a single campaign with one adventure and up to three encounters. Upgrading to the premium version with the in-app purchase will remove these restrictions..