The Celestionic Sector is located in the northern reaches of the Ultima Segmentum. This sector is not well known as it is highly unstable around it's borders due to the remnants of a warp storm conjured during the Horus Heresy.
In the present day, this sector houses the Bullfrogs, Preachers, Celestial Hawks, and Blood Hammers Space Marine Chapters that protect it from the many foul powers that are constantly attempting to secure it's Forgeworlds and vast amounts of minerals. Among these powers are the Eldar, Orks, Necrons, Chaos, and several renegade Human empires.
When this sector was established during the Dark Age of Technology it was discovered to be rich in minerals and created much wealth for those that colonized it's planets. When the Age of Strife occurred the systems were cut off from the rest of humanity by the massive warp storms that occurred. Somehow the systems were able to maintain contact with each other and able to form an alliance that protected each other from attacks by disgruntled planetary populations and xenos.
After the Strife
When the warp storms abated the Celestionic Sector (named after Celestion, the capital of the sector) began to tentatively ply the outer reaches of their sector to find the remnants of humanity. Soon they stumbled upon the Great Crusade and quickly established themselves as great trade partners with the Imperium. The Mechanicus soon took interest in the system for it's vast mineral deposits and created several Forgeworlds to produce materiel for the Imperial Army. For the last 450 years of the Great Crusade the Celestionic Sector enjoyed prosperity and relative peace. Soon, that was to change.
When the Horus Heresy erupted and the Ruinstorm was conjured the Celestionic Sector was caught in the middle. A storm materialized on top of them, trapping them in a deadly storm. Dozens of planets were lost to the Warp, never again to be heard from. But still the planets persisted. Enacting long dormant plans for an occasion such as this they sealed most of their population in shelters that sealed them from the Warp and it's foul powers. For the seven years of the Heresy and the seven years of the Scouring the storm persisted. But at the end of the Scouring the storm mostly abated, still trapping at least thirty planets inside it, but leaving the majority of the planets broken and scarred, but still alive.
To the Present
When the call for the Imperial Guard went out the Celestionic Sector was one of the first to contribute. They raised twenty regiments from various worlds. These regiments became the first of the so called "Star Walkers"; regiments raised to move across the Imperium and gain glory.
In the 32nd Millennium two Chapters were assigned to the Celestionic Sector. First came the Preachers which, have made their base upon an unknown planet. Next came the Celestial Hawks, which liberated the Chaven system and took Chaven as their recruiting world. Then the Blood Hammers came in 450.M36 seeking a stable supply of recruits. They were granted the world of Bli'Carv and they quickly took to it. Finally the cursed Bullfrogs came in 480.M36. They settled on Brazili in an act of desperation and came out bloodied, but powerful.
These forces have served to stem the tide of foul that constantly threatens to overrun the sector and it's planets.