Like being physically out of reach or having a large organisation as protection or an unknown place of residence...
I agree, but all Dark Lords, even the high level "just another mage" casters has it. Vecna in Cavitius has a army of "zealot followers". Strahd has a lot of "creatures of the night", Meredoth has a lot of really nasty traps, constructs and undeads. Most parties on 2E die without even reaching the dark lord.
Even if you somehow exploit the weakness of Strahd(his love iwth Tatiana) and somehow lure him to a trap and kill him, he can just assume mistform and "reform" on his coffin few moments later enraged. The best way to deal with dark lords is to exploit some obsession of then. And still extremely hard.
That's what you took away from Bram Stoker's Dracula that being a vampire is about living in a castle and having powerful spells? Not about being a parasitic monster that likes to play mind games?
Nope. By Mystra, You guys like to distort everything that I say... I just mentioned some this traits to say that Dracula, the most popular vampire is akin to a high level caster, never said that
"everything about dracula is his awesome spells". Even before he become a vampire, he was a caster.
On chapter 23 of the classic novel
"He dared even to attend the Scholomance, and there was no branch of knowledge of his time that he did not essay."
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But all of this discussion only because you distorted
"high level necromancers would't be satisfying villains on 5E" to
"if a enemy doesn't have a lot of powerful spells, he doesn't worth" which I never said. There are countless of terrifying dark lords that aren't casters and are great. Lord Soth is IMO the most iconic Death Knight ever.
https://en.wikipedia.org/wiki/Lord_Soth
And has a very rich story.