I got this weird message popup when I completed the totem puzzle in Yngol Barrow. Right when I activated the gate lever there was a message box that said...

Congratulations!! You placed the vessel.

You get a prize!!

I didn't see anything get added to inventory. I've never seen this before so I tried searching it and found nothing, not even UESP mentions it. Anyone know anything about it?

I know for a fact that the only thing I've done recently involving anything called "vessel" is I completed the Evil In Waiting quest at Valthume, the one where you find the 3 Opaque Vessels to help defeat Hevnoraak. And really, the only mod I have that I can think of that might do something like that is Interesting NPCs, otherwise the rest are all just new dungeons and follower mods. I'm intrigued by it, but at the same time absolutely, completely stumped lol.

Did you check the quest?

I'd suggest using TES5Edit to look at the quest and the lever, and see what mod(s) changed them. You might also be able to find the message itself, but that's more a case of "only when you know where to look".

I never even thought of TES5Edit Thanks for reminding me that this would be a perfect use case scenario.