The following comes from the Warhammer-community page.
Another new bit of Warhammer 40,000 lore from the new edition for you today.
The Damocles Gulf has been a hotly contested region for the century, with the T’au Empire seeking to expand its territory and locking horns with well established, but hard-pressed Imperial forces. The war has been bloody on both sides with the death of a Space Marine Chapter Master answered with the assassination of key Ethereal personnel – there’s a lot of bad blood.
The Imperium, seeking a swift resolution to the conflict in the face of mounting pressures elsewhere, used a pretty drastic solution at the finale of the last campaign – launching world-ending incendiaries into the nebulae of gases across the war zone, effectively setting fire to the entire Damocles Gulf and denying it to both sides!
Problem solved, right?
The Cicatrix Maledictum, the great warp rift that split the galaxy in half, snuffed out the multi-system-spanning firestorm – the war for the Damocles Gulf is back on! With the Imperial defenders pressed hard on every front from Chaos incursions, reemergent Necron dynasties, and ravenous Tyranid hive fleets, the T’au were quick to react to the new opportunity. They launched their Fifth Sphere Expansion to fight back against these terrors of the galaxy, seeking out new worlds and claiming more territory for the Greater Good.
Wait, fifth? What happened to number four?
Yeah, about that. So, the dynamic and ever optimistic T’au might have been a little over-eager to embark on a new expansion.
The Fourth Sphere Expansion fleet set off almost immediately once the fires in the Gulf had subsided, but with the Great Rift still raging through that region of space, all contact with them was soon lost. Were they all killed by some unknown attacker(s)? Did they just lose contact due to interference? Perhaps they entered the Warp – intentionally or not – becoming lost (or worse) in the violent empyrean tides? Maybe they will yet be found, or emerge somewhere unexpected in the galaxy, a lost portion of this optimistic new empire ever intent on conquest…
We’ll have to wait and see.
Tomorrow, we’ll have more from the new Warhammer 40,000 rules when we look at Stratagems.