You need to collect 30 hero stars to unlock Zombie Frank
To feast on humans, you need to use your skill move on human enemies. This can be done by pressing
with a full combo meter. To quickly build up your combo meter, the easiest way is to stun hordes of zombies with the
attack and then feast on nearby humans.
If you are struggling to find humans, you can look around the stores and buildings in each map, and odds are you'll find a couple groups. You can also search for encounters that take place with hostile survivors, obscuris soldiers, or maniacs.
Here is a visual guide for any that may prefer one:
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