It's basically an automatic group-oriented players vs monsters/MVPs event. There's a map most private servers usually use for it, with a big center and 8 narrow corridors going in each direction, which you can access through portals. So there's usually 9 waves of monsters/MVPs, each on a different corridor going clock-wise, until the last one is on the big area in the center. Each wave would increase in difficulty, including MVPs from the 3~4 round onward. They could have "themes" such as elements or races as well, or simply be selection of monsters. All players would be allowed to participate, but access would be closed after it begins. It would happen 1~3 times a week.
There's no actual end game content at this episode, besides FE WoE and.. well, you can MVP, but there aren't really "strong" or challenging MVPs/maps yet (thor volcano, nameless island, biolabs, etc...). Most people are really looking forward to transclasses and a few extra episodes of content. This would allow a new "challenge" / "distraction" kind of thing PVM-wise. It should attract bored veteran players (for the loot/final reward and fun) and fresh players alike (decent/good exp and also loot/rewards).
Open for debate what would motivate people to go there and spend time/stuffs: exp/drops through the event, treasure boxes for whoever stays alive in the end, server-wide boosts, guild rewards, individual rewards, etc (we wouldn't want to break the economy, but without drops or rewards it would be pointless, specially taking the frequency in consideration).
Some possible customizations:
- Putting an entrance fee of 15~100k.
- Making it so no one can join after the event begins (it would open the map and allow people to enter for, say, 30~5 minutes before it actually begins). Or making entrance still possible up to the Xth round, but with an increasingly higher entrance fee.
- Making it so, like PVP maps, after someone dies a second time they are instantly kicked from the map.
- Making the portals appear in sequence and without a portal back, in such way that people get "trapped" on the corridors with monsters, or left behind if AFK. This way people couldn't AFK in a safe place until the end for possible rewards nor abuse going back and forth a portal to GFist and come back or wait for the mobs to disperse before entering again. (Example: 1st corridor is cleared, a portal to the start of the 2nd opens by the end of it. The moment someone passes it, monster spawns of the next round start. When the last monster dies, the portal on the 1st corridor vanishes and a portal to the 3rd corridor appears by the end of the 2nd. [...])
- There could be a "random" mob spawn on the event that would be each MTOD.
- Could entirely remove the EXP from the mobs and make it only loot-oriented.
- It can have fewer rounds, the 9 mentioned, or even more...
- How difficult (monster strength, mob density, ammount of and which MVPs) is entirely up for debate, as in if a small group of people will be able to clear it or if it would require several parties and a server-wide effort to actually clear it all the way through...
- Depending on how difficult it's set to be, there are three possible extra rewards: some treasure chests spawning on the map for the "winners" (open for debate what would be inside those and the drop rates), individual rewards for each player still alive delivered by the exit NPC, and/or maybe triggering a +5~50% EXP and/or DROP rates boost for X hours.
- PVP or WOE-PVP could be activated at some point, like in the final round or after the final round and before treasure chests spawn.
- Alternatively you could simply customize Endless Tower for this episode, or do anything else along these lines in other maps (spawns of monsters and a handful of MVPs).
(images for map-wise illustrative purposes - got from a random video on youtube)
TL;DR: Waves of monsters and MVPs on a special map for possible rewards if cleared, or simply the reward being the exp/drops/fun of clearing the mobs all way through. No need for GMs, automatically happens x times a week.