The book was originally written for Pathfinder, and we are adding 5e language and rules to it. Where needed, both sets of rules are presented, with Pathfinder specific text in a different color. We just used this technique successfully for our project The Dread House which blended 5e, Pathfinder, and Call of Cthulhu into one book. Beyond that, this is a dungeon crawl, meant to be easily modified, with monsters, treasures, and quests being interchangeable as the DM/GM desires. That makes it easy to port it's content into other systems.