The Celestial Lions is a Loyalist Successor Chapter of the stoic Imperial Fists, raised in the 38th Millennium during an unknown Founding, though some Imperial scholars suspect it may have been the 23rd Founding.
Following the 25th Founding at some time in the latter half of the 40th Millennium, the Celestial Lions became a part of a triumvirate of Space Marine Chapters which included the Emperor's Spears and the now-extinct Star Scorpions, as a part of the Adeptus Vaelarii, better known as the "Sentinels of the Veil." These three Chapters were charged with the sacred duty to stand sentinel over the scattered worlds of the region of space known as Elara's Veil.
Since its inception, the Chapter has had a long and glorious history, stretching back over 4,000 Terran years, but events set in motion over five solar decades ago during a routine assault on the world of Khattar in 948.M41 nearly proved to be the Chapter's undoing in the wake of the Third War for Armageddon, from which less than a company of Celestial Lions emerged intact.
Targeted by a faction of the Inquisition to serve as an example of what happened to an Adeptus Astartes Chapter that stood against them in Imperial politics, the Celestial Lions survived only due to the intervention of the Black Templars, fellow scions of their Primarch Rogal Dorn. The few surviving Celestial Lions returned to their homeworld of Elysium IX in Elara's Veil under the command of their new Chapter Master Ekene Dubaku to begin the painstaking process of rebuilding their Chapter's numbers back to full strength.
Later, after the birth of the Great Rift, the Chapter became involved alongside the Emperor's Spears with defending the Veil from an assault by a great Chaos warhost called the Exilarchy. The Exilarchy was under the command of a warband of Heretic Astartes called The Pure, who were the Chaos-corrupted remnants of the Star Scorpions Chapter that had once been a part of the Adeptus Vaelarii.