ive been thinking about the sorta research path in my lich game concept that would lead to the development of death knights as your lategame elite soldiers
so, it'd start around midgame, with a group of alchemists you've got working for you asking that you use a device they invented to extract the essences of powerful monsters for experimentation. you do this, and you get a new action that can be performed in the alchemy lab - essence imbuement.
essence imbuement involves taking a subject and basically pumping a bunch of the essence of a given monster into it - initially, this would allow for things like giving a particular type of unit more health or higher resistance to a given damage type etc.
then, you try imbuing multiple essences to a single subject and... it fails. the subject just sort of melts. so you have to research some way to stabilize it, this takes a while.
when you finally find a way to stabilize it so your unit doesn't just melt, it instead causes them to die - which, no matter. you're a powerful necromancer anyway. you try to raise them normally and they just collapse back to being inanimate. it doesn't work.
finally, you figure out a way to keep them from dying - undeath essence, harvested from your own eternally undying form. by adding that to the mix, you can combine as many essences as you want in a given subject, allowing you to optimize all of your units.
then, there's a final step - imbuing essences not across a class of units, but into a single individual. the result of this being your death knight.