From publisher blurb:
Use your favorite monsters but change the game!
Make your monster new again without changing its Challenge Rating. No templates to apply, just switch out the monsters existing abilities for the new ones. This saves you time while instilling a sense of dread and wonder of the unknown back into the game, even for those jaded players who have memorized the Bestiary. Do more than just make cosmetic changes, use a variant monster whose new abilities transform the very nature of encounters. However, don’t think we have forgotten the alterations to appearance, habitat, and behavior, because want to inspire truly evocative and innovative encounters.
From the lowliest Brownie to the Mighty Tarrasque, this book covers 5 variants for each CR from 1-20, plus a CR 25, which makes it useful for Game Masters at all levels of play.