Best Monster/Boss for a One-Shot for 3-4 3rd level Characters.

[SeasideDruid]

Im currently working on a one-shot and the final boss is a powerful undead warrior king. I'd like him to be sort of a towering Dark Souls style kinda monster so I'm coming here to ask which monster/stat block could I possibly re-skin for this boss cause I'm not smart enough to come up with my own stats.

[Yora]

How about a Helmed Horror? All by itself a 3rd level party should have a chance to beat it, and with it being a one-shot there is no problem with some or even all of them possibly dying.

[Specter]

You could take the Veteran from the NPCs in Monster Manual and give it some template (vampire, wight, etc.), or take a wight and increase HP and damage. The most important things would be undead nature (no food, air, sleep required), some resistance to necrotic damage, sunlight sensitivity and immunity to poison.