Here's some info about the Under the Castle event from the player point of view. I have made some images that show what it really looks like when played, not like the trailer videos.
I'll just link to the pics instead of embedding them directly, since they are large and there are too many of them to dump all here. You can view all of them at once here: http://kingofknight.com/images/underthecastle/
- The first thing to note is that death works very differently from dying in the normal zones. You don't lose any EXP, but all of your items will instantly become red (0 durability). There is no rezz (skill or scroll), the only option you have is using a Crystal of Life which cancels out the item-breaking effect and also resurrects you on the spot rather than in the bases. Considering that TP/friend scroll etc. also don't work, this is another important part, since you have to run all the way from the base to the current stage, and at the 4th stage you can easily miss the boss being killed. You can get a Crystal of Life from the entry NPC (Ireya), but only 3 pieces per day. It's also supposed to drop from Item Disassembly.
- Currently there's no official timing for the event. You can enter the map at any time, but it's useless when the event is not in progress. When it starts, everyone is instantly teleported out to the mainland. There's also no notice about the event starting. However, I have seen some mobs inside before the bosses spawned.
- Bosses have 3 or 4 health bars. It works like some fighting games: first the health bar is teal, and as you damage it, it shrinks to reveal a yellow bar. Then the yellow bar shrinks to reveal a blue bar. Then again, revealing a red bar. The red bar is the last one. Each time a bar is depleted, there's a bright flash on the screen (even if you are far from the scene), and sometimes a video is played.
- The first boss is Turkish-themed. His name is Mehmed III. He keeps summoning minions like Ceri, Sipahi and Kapikulu. He also casts nova-like AOE's around the room. When the event starts, all the weak players are conglomerated in the corridor to avoid the AOE's while the strong players kill the boss. The other bosses also work in a similar manner.
- After Mehmed is defeated, a new boss appears in the same room, named Crusher Gimmick. He's a giant robot. Sometimes he spawns Proto-Gimmick MkI and MkII.
- The bosses don't drop loot directly. Instead a lot of boxes appear at the location where it was killed. (Mehmed doesn't spawn these.) You must attack the boxes to get the drop (not sure if this has to be R-attack or skills are allowed, as my char is too weak to endure the AOE's.)
- After the first room there is a winding hallway with some mobs on it. This one is called Moebius Arc.
- The next boss is [Rainbow of the Abyss] Furious. He has an upside-down human face on his belly, and has multiple tentacles. He spawns three-headed lizard mobs and the Moebius Arc seen above.
- After Furious you fight the chained Destroyer Pluton. He's a huge winged demon and he appears on the login screen background. Mob spawns, big AOE's, you know the drill.
- Finally, you pass through some kind of portal and arrive in a circular room where the freed Pluton awaits (this time he's called Hell Destroyer Pluton). Yet more mob spawns and big AOE's. One of the mobs is called Iblis.
- Each time a boss dies, you automatically receive one of the four items for the King of Kings quest. You must be near the boss; if he's not on screen, you might miss the item. If you have all four, you can turn them in once per day for 3m EXP.
- Other items drop from the boxes, which I haven't managed to open yet.
- After defeating the Hell version of Pluton, 2 NPC's appear in that circular room. They have identical functions. They give you 50 NP and a Rising Scroll (+30% EXP for 1 hour or such) once per day. They also exchange some of the advanced Booties, which I suspect are a box-dropped item.
- Finally, something that's not quite related, but deserves mentioning. Whenever a player gets a unique item, it gets announced in the chat window and on the top of the screen. Also if someone upgrades a unique to +7 or higher, or an accessory to +1 or higher, it is announced, and so is if the item in question burns! The following image shows some of those notices including someone burning a +7 sherion. The item names are readable because they were translated in the English Patch for the old version.