Gaerzi: 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.

tinyE: I'm still a little vague on how this enters into the game. Is is pre Hardmode, Hardmode? Is it unlocked when I find something? Does the new NPC show up when a certain event happens? Does all the new stuff happen in sync with the classic "fight the Corruption/Crimson" plot line?

Gaerzi: 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.

Gaerzi: If you try on a fresh world, keep in mind that you need to have already defeated the Brain of Cthulhu or the Eater of Worlds.

As for the seed, it's just a random seed. When a world is generated from now on, its seed will be stored. Then you can use that seed to regenerate exactly the same world. That way you can share a fresh world easily, or regenerate it to its original condition. Since it only allows to redo the original random generation identically, any change to the world (like cutting down trees, digging tunnels, building houses, or moving spawn points) will obviously not be "saved" in the seed.