In each of the nine main levels there is a hidden skull called "Imbued Remains" that increases your max HP by 5 for a total of 145 if you get them all. Here's where to find them.

Spoiler Warning!
This guide will give away secrets so proceed at your own risk.

Once you get to the road by the big bridge over head jump into the pond and look for the sewer pipe under water.







The remains are at the end.

At the docks beside the big ship there's an upside-down rowboat.



The Skull is underneath.

Midway through the level once you enter the town head over to the lunch hall and enter the basement.



After you kill the Brute down here turn around and find the destructible wall.




Blow it open for the Skull.

About halfway in the level you'll drop into an underground lake.



Look in the sunken mine cart for this Skull.

You'll come through large sunken room full of zombies.



The skull is in a small hole in the wall on the right side of the room.

The Remains are kept in a small nook on the right in the basement of the chapel.
Pretty hard to miss.



WARNING!
If you leave the basement without picking it up you can't re-enter. The Basement door locks behind you.

The skull is pretty far in the level. Once you make it up to the roof of the asylum head past the tower then turn around and look for this tiny platform.





The skull is right around the corner.

Play through the level until the 2nd time you enter the crawl space inside the walls.



climb up to where the two axe cultists ambush you in the little kitchen.



Then jump up two more platforms to find the Remains.

Deep in the level you'll come to a giant temple. Go into the temple then head down the big spiral stairs to this cross walk area.



Turn left and enter the Library. The skull is around the other side of the circle bookcase.

That's all the hidden skulls in chapter 1. I'll update this guide once chapter 2 comes out and I find them all.

Feel free to comment and let me know if I need to add more screenshots to help make finding the skulls easier.

Happy hunting Detectives!