You need to find the new NPC. He can be anywhere. People have reported finding him on the surface near their spawn, others said he was in the underground. If you have a detector it'll appear as "unconscious man".

The tavernkeep sells a crystal and related stuff. You use the crystal to trigger the event. The crystal must be placed in a mostly flat, open area, so you can't cheese it too much by putting it behind a wall.

The event can be triggered at any point during the game progression. The enemies exist in several variants, so you'll get easy variants in early game and hard variants in late game.

It is not directly related to Corruption and Crimson. It's a different threat, an invasion from a different dimension.