The Blood Games are the military exercises of the Adeptus Custodes by which individual Custodians seek to test the defences of the Imperial Palace on Terra by penetrating its security. These exercises allowed the Custodes to anticipate all of a potential enemy's own designs and strategems so that they could harden their own defences and better protect the life of the Emperor of Mankind.
Between the time of the Horus Heresy in the early 31st Millennium and the start of the Indomitus Crusade at the end of the 41st Millennium, a specialised corps of the Adeptus Custodes was tasked with finding suitably powerful foes, including Heretic Astartes, who were taken from across the galaxy to be released within the Imperial Palace complex as part of another Blood Games exercise to test the Custodians' abilities. This was necessary because until the opening of the Great Rift, the majority of Custodians never left the grounds of the Imperial Palace.
During the Invasion of Ultramar in 999.M41 when the Bloodborn assaulted the world of Calth, the Chaplain of the Ultramarines 4th Company, Judd Clusel, suggested to his commander, Captain Uriel Ventris, that he try to emulate the enemy just as the Custodes did during their Blood Games, to determine where the weaknesses in Calth's defences lay. This indicates that the Custodians' practice was known outside their organisation, at least among the Adeptus Astartes.
- The Chapter's Due (Novel) by Graham McNeill, pg. 180
- Blood Games (Short Story) by Dan Abnett
- Watchers of the Throne: The Emperor's Legion (Novel) by Chris Wraight, Ch. 2