Be warned, most of this stuff is basically amalgamated from Warhammer wiki, Lexicanum and 1d4chan.
Content purity is anywhere from skeptical to heretical. Damn sight better than 'Speech To Text Machine Series' though. (Yes, I'm bitter about what they did to my darling Ksons)
1. Currently in lore Ghazkull is in two different time-thingos.
a) He's gathering up all of the orc hordes Beast- style and he plans to use the collective Waagh to awaken Gork and Mork as corporeal gods in their own right.
b) He's fleeing Commisar Yarrick and the Black Templars after the Armageddon II postwar stuff.
2. Necrons (VERY RECENTLY) have learned how to use the webway and are using it to their full advantage in order to rip suckas up. They would be the strongest force in the galaxy except for that one thing they are missing, which is a soul, which allows them to Warpstuff. They're deader than the Tau when it comes to psychics - ergo they are terrified of psykers.
Is why they created Pariahs and those big "Contain the eye of Terror" pillars on Cadia.
3. After the Pyrrhic victory of Chaos over Craftword Ulthwe and the Imperium of man in the current Black Crusade (
To-date the only even marginally successful one), the Chaosy armies currently hold the majority of Cadia, whilst the Imperium have them locked there.
Which you would think would be an acceptable stalemate except that all of their foul decadent rites are cracking those "Contain the eye of terror" pillars.
Exterminatus (I can only assume) would only mean backwards steps for the backwards empire. So who knows.
Point Being:
Could Ghazkull's upcoming WAAAGH! potentially invade and destroy the Eye of Terror?
That isn't the purposes of the Orcs. In fact, if they were to move into the Eye of Terror, they'd probably have an even stronger Waaagh, and may in fact succeed in their 'Creation of new Chaos Gods' thing. (It's iffy on whether or not Gork and Mork inhabit the warp. They're a complicated duo).
Point being, if they don't, they would, and if they have a nice little birth like Slaanesh, that's only gonna rile up the warp more... Which is bad for the Imperium.
Orky Demons. Be afraid.