I like this recent 13thAge-compatible DIY monster tool from Rite Publishing. The book has a not so simple origin story. It first came out in 2011 as a Pathfinder sourcebook written by Steven D. Russell. Step forward a few years and Patryk Adamski approached Steven and Rite Publishing with a reworked 13th Age compatible version. The mechanics are new, the art is new. Like Kobold Press’ and ASH LAW’s Deep Magic volume that's compatible with 13th Age, 101 Not So Simple Monster Templates is a book that is inspired by the previous Pathfinder RPG edition rather than a straight conversion which is confined to the original mechanics.
What you get from this Rite Publishing book is an alphabetical list of 101 conversion templates for customizing monsters on the fly. Many of the templates add a level to the monster, so that the impact of the template’s new abilities and powers get offset by lower stats. A few of the templates, like Burned Out Creature or Unhinged Creature, go the other way and reduce the creature’s effective level.
There are a couple niggling problems. A few of the templates use language that’s different from standard 13th Age terminology, but not so different that it’s difficult to figure out. A few other templates almost certainly err on the side of being too nasty. Resilient, for example, has got to be missing its level adjustment.
But balance issues are minor, particularly in a system that advises GMs to regularly make battles unfair! If you’ve been running 13th Age, you’re going to be able to recognize the few too-nasty templates easily, they’re not subtle.
I’m especially happy with 101 Not So Simple Monster Templates because its text is all published under the OGL. As a designer, I’m not likely to borrow a full template and the template approach, but there are several creative mechanics here that I’ve already borrowed or revised as elements in new monsters headed into 13th Age in Glorantha and future installments of the 13th Age Monthly. So I’ll be adding this to the list of OGL books in the licensing section of an upcoming product or three.