The Adeptus Astartes otherwise know as Space Marines collectively known as Angels of Death are the most elite and feared fighting forces of the Imperium.
The Primarchs were genetically engineered from the DNA of the Emperor himself. After their creation they were lost to the Emperor and scattered throughout the cosmos by the forces of Chaos.
The Space Marines came into being when the Emperor created them from the genetic material of the Primarchs after they were lost. These Legions took part in the Great Crusade, during which the lost Primarchs were rediscovered and given command of their respective Legion.
The Blood Angels, under the command of their Primarch Sanguinius defended the Imperium fighting side by side with the Emperor as he strove to reunite sundered mankind.
During the Battle of Tera the traitor Horus cast down Sanguinius before the Emperor finally vanquished his ‘son’ who was lost to Chaos. The aftermath that followed was great. The Emperors shattered body was taken to his Golden Throne to be sustained by life giving machinery less he perish. Mankind had suffered greatly and the Imperium was nearly destroyed, yet the Blood Angels would bear the pain for longer and more deeply than most in the Flaw.
To learn more about Blood Angles visit the website Lexicanum for lots of information regarding Blood Angels and Space Marines in general.