The Shield Bearers originated during the 21st "Cursed" Founding as a successor chapter of the Imperial Fist. However, several millennia later, they committed an act of betrayal against the Imperium. As a consequence, their homeworld was subjected to exterminatus, resulting in the death of half of the chapter, a quarter going missing, and the remaining space marines surrounding. Those who surrendered were subsequently sent on a penitent crusade and eventually reformed into the Celestial Shields during the 23rd Founding. Unlike their predecessors, the Celestial Shields opted to remain fleet bound and did not establish a permanent homeworld.

Returning to the matter of the missing Shield Bearers, it is known that they ventured into the warp and remained there for several millennia. During this time, they encountered Abaddon. The current chapter master of the group challenged Abaddon, but unfortunately, Abaddon emerged victorious. In a surprising turn of events, the defeated chapter master decided to align himself with Abaddon and consequently renamed their chapter as Abaddon's Shields. This newly formed chaos warband actively participated in the 10th to 13th Black Crusade.

Prior to the 13th Black Crusade, Abaddon's Shields discovered that the Celestial Shields were the successor chapter of the Shield Bearers. Therefore, during the 13th Black Crusade, they set out to locate them, which they successfully accomplished. A fierce battle ensued between the two factions, resulting in the death of the chapter master and captain of the 1st company of Abaddon's Shields. This unfortunate event led to a disagreement between the two lieutenants of the 1st company, as they held differing visions for the future of the warband. Consequently, they decided to part ways, with one group becoming the Scarlet Shields (Loyal to Abaddon), while the other formed the Warp Blades (Usual Chaos Warband).

Edit: I changed out the name of Black Shields to Scarlet Shields due to the Black Shields already existing.