Possibly my favorite cosplay project to date! I purchase this template from the awesome folks over at https://www.punishedprops.com/
This hammer is made from gym-grade foam sheeting and PVC pipe.
Even the leather wrapped handle is actually 2mm craft foam!
The finish is a combination of techniques including using cotton batting and liquid latex to create a muddy, matted look. The wooden portion of the handle is actually wood. I re-sawed a piece of scrap down to about 1/16" thick, then soaked it in warm water, wrapped it around the PVC pipe, and let it dry. Once dry, I measured and splice cut, then glued the wood onto the handle with CA glue.
Here are some additional pictures of this project.