This is just a little idea I dreamed up. EDITED April 8th 2014
Story: The Long Migration was the most mysterious thing since the Skylark crashed on Cradle. Now we think we know who called the Tortodrones on their migration. It was the Apocrea! They are assembling a Tortodone army to capture all the souls on Cradle. Spiral HQ has tasked Lieutenant Feron with stopping this plot.
If it's an event, you're in Tier 3 and get sucked into Apocrea like with the mission. If it's an expansion mission, you go straight to the start.
Lobby: Obsidian Nexus with Feron and other knights camped out. You can talk to Feron for some more in-depth information.
Level 1: Grasping Plateau with 2 elevators. You can only take one. One will go deeper into Apocrea and one will go through an Apocrea-themed Unknown Passage before meeting up with the first. You can't split.
Level 2: A graveyard where it seems the bodies have been deposited. The phantoms here are Apocrea themed. Each grave either has a Carnavon or a freed knight (NPCs).
Monster: Apocrean Phantom
Looks like a knight in Apocrea costume. Wields an Obsidian Edge. There's one per knight. They appear almost everywhere in the mission.
Level 3: A crazily constructed city that has dozens of Apocrea (multiple reskinned Harvesters). Also many Apocrean Phantoms. Some arenas.
Level 4: Obsidian's Palace. More Apocrea Phantoms but no reskinned Harvesters.
Level 5: Boss battle level: Obsidian
5 phases
Note: There are always Apocrean Phantoms
Phase 1 you defeat some Carnavons
Phase 2 you fight Obsidian
Phase 3 Even more carnavons and some more Apocrean Phantoms
Phase 4 Obsidian again
Phase 5 Obsidian, with a really large amount of Carnavons and some Tortodrones
Obsidian has all the attacks of an Apocrean Harvester, plus the teleportation. He can also summon bullets that automatically target knights.
End: Obsidian Nexus.
Please help me make this better.
Heh, I made a rhyme
This sounds really neat. I like the idea of the two elevators and not knowing which is which. I wouldn't say defeat so much as I would have a sequence in which players have an apocrean-themed unknown passage where they have to stay alive, as "this isn't the correct way - escape!" This would make the player not feel like they have fallen to the harvesters beyond their own control, and instead must escape, which if they can survive, will not be rewarding other than the experience (perhaps there can be a single apocrean material at the end with a really crummy RNG behind it - 20% for the 3* material, 3% for the 4* material, and 1% for the 5* material. This is to prevent farming for materials while the event is not in progress but make the escape mission not feel like its worth nothing)
Throwing tortodrones into the mix may be a bit harsh - I would love it, but it doesn't fit into the apocrean lore, unless we have fiends in places every-so-often, in which case corrupted tortodrones (which we've fought recently) can show up. Make sure to have corrupted tortodrone bodies laying around, and have it be a single tortodrone in a wave, else have one tortodrone and maybe two apocrean phantoms that don't respawn (see my comments below). Don't have it be in the final boss arena though - make that fight its own unique thing.
A thought on phantoms - Give these ones more health than regular phantoms, different sounds, and longer respawn times. The ones in the city should not respawn, and instead should take the spot of an "elite" enemy (I.E., mecha knights, gremlin thwackers, grievers). They should also have a blue or purple personal color, and have the apocrean crown and disciple wings.