From the publisher's website:
This is no ordinary bestiary. While of course it includes a host of new creatures, it’s more than stat blocks and bland descriptions. It gleams with the essence of 13th Age, with the same quirkiness, the same match between flavour and mechanics you saw in the core rules.
Find out how each creature relates to the icons, make use of multiple adventures hooks for every creature and use nasty specials and variations to keep your 13th Age campaign unique.