This tool is called an adze, it is just like an axe but the cutting edge is at 90 degrees to the handle. Adzes come either with straight blades for creating flat surfaces or curved ones for hollowing. This is a very old one I bought in Romania in 1998 and works excedingly well. I use it every time I make a plate to hollow a little of the wood before I start turning. Curved adzes are very hard to come by in the UK but straight ones are quite common. I have a few old rusty straight ones and really need one with quite a deep hollow for hollowing the gutters I want to make for my new barn. So taday I spent the morning in the forge.
I had never understood why hand forged adzes were so much more expensive than axes when they look fairly similar. I had not realised the complexities of creating those two even curves in two planes and if either curve is just not perfect the tool will not work properly. I hope mine works OK.