Military action
The First Horizon’s fleets enter the Third Horizon. The Portal Wars begin. The old factions are primarily fighting the Empire, but also each other in confusing side conflicts. Many portals are damaged by the unchecked use of thermonuclear and antimatter-based weapon systems.
Eventually, the First Horizon wanted to reclaim the systems that had freed themselves. The newborn separatist powers, headed by the Order of the Pariah, resisted, and war broke out. The slaughter lasted for several years, but ended when the star fleets of the Third Horizon finally wiped out the First Horizon’s lackeys – a faction called the Sacrifice of Nazareem – and closed all portals to the other horizons. The terrible war left in its wake remnants of once great armadas, rifts in the fabric of spacetime, and cities, whole planets even, scorched and destroyed.