I didn't expect that one :)

Definitely didn't expect seeing these two games together

When I click the link to download the "Visitors from Morta" DLC, I get the installer for the "Paint of War" DLC instead. Anyone else has the same problem?

Same here, will likely get fixed later today.

While I dislike most crossovers on principle, this is the way to add such a thing to a pre-existing game: an optional bit of DLC that one needn't even download if one doesn't like or care about the other thing being referenced. Not in an update to the main game, mixed in with a bunch of other general improvements, with no way to disable the extraneous crossover stuff, and leaving one with the odious choice between updating and gritting one's teeth through the fan-fiction-y bullshit, or staying on an older version and forgoing any bug fixes, quality-of-life improvements and such that came with the promotional crossover update. (Looking at you, Terraria. >:( )

Indeed, this has been fixed :)

Does anyone know what exactly the two new archetypes are?

I hear one of them is a witch with scaling offensive cantrips.