Legendary Monster is for those "Holy crap how did it do that?" moments
I don't see how that supports the idea that undead creatures cannot be legendary monsters.
And even more templates specifically don't work on undeads/constructs while working on pretty much everybody else.
It doesn't change that my statement is quite valid. Undead creatures are not static and come in many flavors. Their very nature makes them easy to warp and subjective to produce unusual specimens.
And as Raineh said, most of the templates you speak of are inherited anyway, which must be acquired before they can become undead. And for those you have a tendency to make them available anyway via rituals or some other mean.
Several undead types auto-discard any templates you had beforehand.
Those are mostly templates changing the body into an entire new creature, which makes sense that the entire creature is discarded to make the new one. Usually those are meant to make minion-material undead that aren't meant to be played by PCs.
And ironically enough, undead-turning templates don't work on undeads themselves.
While some combination being impossible might make sense (skeleton+zombie?), I agree that some should technically be possible and perhaps weren't because the devs kept to the pattern that a template granting undeath should require the base creature to be alive. Still, you do not limit yourself to such constraint so nothing stops you from allowing vampires or any other undead creature from becoming a lich.
If it has a solid body but no traces of organic matter, it's not really an undead anymore is it?
Your only counterargument is nitpicking at an example? If you must, I'll remind you that bones are made of carbon, which is the same element from which diamond is made. I doesn't change much. Monster of Legend already makes it possible to change your body to weird stuff for aesthetic purposes only.
Either way, my point stands; them sticking to their role or not isn't relevant in whether they can be legendary or not. Getting crazy powers normally unseen by their kind is very possible to that creature type
I don't give a crap about such "vampires" either.
Me neither. So what? You used sun-light and vampire for your example - I refer to undead creatures in general even though people here mostly think of legendary vampires. A vampire could have a different crazy ability unrelated to sunlight. Like lava blood or whatever.
They're known as flesh golems and animated objects. You know, the ones that don't specifically block the people used up in te process from being reincarnated.
I do know. I was just giving information on the matter of walking bodies with a soul compared to walking undead bodies with one since the topic was brought up. Wasn't particularly directed for your attention.
Thanks for listing examples.
