From publisher blurb:
Need a new monster to put fear back into the hearts of your players!? BEHOLD, the Cryptborne. This undead creature is actually two individual creatures in one, with a Cryptborne Symbiote inhabiting the undead husk of a creature that it germinated within.
This unique relationship creates a creature that is a unique challenge to players both old and new. The creature, while not especially powerful, excels with additional action economy and spellcasting ability not found in lower orders of undead. Of course, the symbiote that inhabits it's chest is a tempting target...and a terrible weakness.