Spoilers ahead

Have you found a way to get more than one King's soul in one playthrough? I mean Vendrick; not the Iron King.

Keep in mind the soul doesn't drop from Vendrick himself.

I've used ascetics on Vendrick's area and fought him again to be sure, and even went and beat the last bosses again, then went to the shrine of Amana and asceticed that bonfire, but the chair with his soul was still empty.

Seems like it isn't possible, but I really want that sword to match my shield and armor without a whole other playthrough. Anyone have any luck with this? I'm afraid I already know the answer, but I'm hoping I'm wrong.

I doubt it. It's the only reason I can think they would separate a boss from its soul drop.

Is it possible you're leveling up the wrong bonfire? I know I upgraded one bonfire once and it didn't respawn the enemies I thought it would. Maybe try upgrading all the Shrine bonfires.

Some drops (such as keys) are considered unique to the game cycle. You simply can't get those items twice no matter how many ascetics you use until you actually start the next game cycle. I assume the King's loot is one of those once a run items.

@karkarov: yeah...