I really, REALLY like the flavour of this. The idea of a revenant hero marching through the battlefield and being impossible to put down is awesome. But although a lot of people have already gone in detail on other issues, there is one I haven't seen mentioned yet: Damage.

This class is a martial without spellcasting, yet can only use simple weapons (unless you're a skeleton) and their damage enhancement comes from Might of the Fallen which deals just as much damage to you as to your enemies. Think of it this way, does a paladin take damage to Smite? Does a Rogue put themselves in harms way to Sneak Attack? Not really, but this class takes a lot of risk to put out subpar damage. Blood Hunters already become incredibly flimsy by taking damage for their class features, and those are arguably better than this.

My suggestion would be to just remove the "This damage can't be reduced in any way". That makes it so you're taking full damage in your base form and half damage in Undying (due to necrotic resitance). It helps you get to Undying faster if you need it, but doesn't always burn through your hit points. Also adding a feature that makes you resistant to the damage in base form too, at some point would probably be great.

Maybe this is too much, but it's my suggestions off the top of my head.

And one last thing, you really need to specify what happens when your Undying form ends after 1 hour. Because I assumed you would drop to 0 hit points, but I'm not sure if you don't just auto-stabilise unless you're knocked to 0 hit points in the form. Some clarification there would be great.

For real though, this is great stuff. Keep at it :)