The tool is a slate (or slater’s) ripper. It removes damaged slates from the roof so they can be replaced. The thin blade reaches under the slate, the hooks at the end grab the nail; the curved part gives you a place to hit with a hammer and cut the nails.
Here’s a simple explanation (there are more detailed ones available too).
https://www.wonkeedonkeetools.co.uk/slaters-ripper/how-to-use-a-slater-s-ripper/