It's kind of pointless leading us on.

What's so important about Arkhona that 5 major chapters, legions, warbands, craftworlds, a tyranid hive, and even MORE in the future all go there to do battle at the same time? This is Black Crusade levels of war if not more that we're talking about here, so I await a good reason.

It's a hunt for Matt Ward obviously, what else could it be, everybody wants to claim the prize

My guess: It's a Tyranid mind control device.

...a la StarCraft 1...

...my god Ivan, that's a terrible idea - RECONSIDER! Don't do it!!!

That's why Grey Knights are not in the game... I mean, they will be, but they are protecting Matt and is our mission to fight them

its because Arkhona was actually the site of the last ever held Adepticon (table top gaming tournament) in the 22nd millenium before GW finally went bankrupt due to its decision to price a single marine miniature at 10 billion Terran credits,

rumour has it that somewhere on Arkhona is a small box containing a mint condition "warhammer 40,000 : rogue trader" signed by Rick Priestly and his actual dice set

one of the greatest pieces of science fiction--no.

one of the greatest pieces of literature ever written in the history of ever.

Obviously like the Dark Angels you have to have earned the right to know certain secrets here on the EC Forum.
I guess if you don't know, then you have yet to earn the right to know.

Inner Circle when

I don't think any of us here are that high.
Plus it's by invitation to join a higher circle.