Lord British's army will be launching a full assault on the Guild of Eternal Shadow. The raid on Granite Castle will last 3 full days, starting Friday and ending sometime on Sunday. During this time, you can expect multiple boss-battle / difficult encounters similar to past events (Drax, Nate, Heftimus)
Below are schematics of the Granite Castle. You can find it's entrance just southeast of Shame's entrance. They share the same side of the mountain range. The secret entrance to the Castle is through the sewer system. Gross.

Work your way through the secret doors and lava passage.

Doing so will bring you to the main room in the castle. A blacksmith shop, a scholar's lounge, a brawling platform, and a hall of Honor to past leaders of the Guild. Continue further and you'll see staircases leading up. Where do they go?

The stairs lead to the throne room of Eternal Shadow. This is where the 3-day event will take place.

The room will spawn monsters frequently. It's a final opportunity to battle with the Guild and any other special monsters from this event series. This includes a rare chance to spawn something extra powerful (eg: Titan Overlords). Boss battles will occur in this room as well. A broadcast will be made anywhere from 30 to 120 minutes prior to the encounter.
There may even be some other surprises involved, and a few final chances to loot one of those prized banners!
Hope to see you all on the battlefield!