So, a front page post about a mystery. How mysterious!
Here's the answer to what The Red Waaagh! is:
It's either a supplement or an Apocalypse warzone.
Check the Timeline in the new Rulebook, fairly near the end. What's that - it's an entry called... called The Red Waaagh? Most of those timeline events now relate to books that are being released (supplements, or Warzones).
Mystery solved. Was that hard? No? k.