I like it, and the eventual ability to sneak attack / crit undead is nice. However, the requisite of improved combat style mastery mean that whoever takes this class must be a ranger, and thus, to get full effectiveness out of the class, means that he must make his initial favored enemies undead and constructs... an unwise choice.

I'd suggest making the improved combat style instead be the equivalent feats, so that, for example, a rogue could take the class with a feat investment.

Also, you might want to clarify if the second level ability stacks with it's Pally equivalent... might get nasty.