The Black Carapace, also known as the Interface, is the last and one of the most important of all the 19 gene-seed organ implants a Space Marine Neophyte will receive as he is transformed from a normal, adolescent human male into a transhuman Astartes.
This neuroreactive, black, organic, fibrous material is implanted directly under the skin of the Neophyte's torso. Points are then cut through the carapace by an Apothecary using surgical tools that allow a Space Marine to directly interface his central nervous system with his Power Armour's cybernetic systems and with the interfaces of certain Adeptus Astartes vehicles.
After a few solar hours, the material hardens and invasive synthetic fiber bundles that will serve as connection points for neurons grow inward and interlink with the newly created Astartes' central nervous system.
Note that a Space Marine needs the Black Carapace to use his Power Armour to its maximum capabilities, but Power Armour technology in general does not require this implant in order to function. The Sisters of Battle, for example, as well as some Inquisitors or wealthy Imperial functionaries such as Rogue Traders, also wear Power Armour into battle.
However, since their central nervous systems are not linked directly to their armour's Machine Spirit (artificial intelligence) as a Space Marine would be, movement is cumbersome in comparison to the surprising grace and natural movement afforded by Astartes Power Armour, and the suit is likewise less capable of efficiently providing bio-monitoring and auto-treatment functions.
Also, while driving or piloting Space Marine armoured vehicles such as Rhinos and Predators, a special spinal interface plugged into Astartes Power Armour through the Black Carapace provides the Space Marine, known as the "Custodian" of the vehicle, with an intuitive "feel" for its systems and controls, literally making him a part of his vehicle.
But this initial design was flawed and had to be perfected by another legendary scientist named Ezekiel Sedayne who had been alive during the Age of Strife. Sedayne ultimately replaced Astarte as the director of the Biotechnical Division.
Ezekiel Sedayne's mind was later digitally combined with that of Archmagos Dominus Belisarius Cawl during the Horus Heresy through the use of Cawl's "soul-merging" technology. This was why Cawl would dimly remember working alongside the Emperor in the creation of the Space Marine gene-seed organs, particularly the Black Carapace, thousands of Terran years later.
- Belisarius Cawl: The Great Work (Novel) by Guy Haley, Ch. 19
- Codex Adeptus Astartes - Space Marines (8th Edition), pp. 10-11
- Codex: Adeptus Mechanicus (8th Edition), "Belisarius Cawl," pg. 39
- Codex: Black Templars (4th Edition), pg. 10
- Codex: Space Marines (5th Edition), "The Making of a Space Marine," pg. 11
- Deathwatch: Core Rulebook (RPG), pg. 18, 161
- Index Astartes I, "Rites of Initiation - The Creation of a Space Marine" by Rick Priestley & Gav Thorpe
- Imperial Armour Volume Two - Space Marines and Forces of the Inquisition, "Organisation of Armoured Units," pp. 9-10
- Warhammer 40,000: Compendium (2nd Edition), "The Origins of the Legiones Astartes" by Rick Priestley, pg. 9
- Warhammer 40,000: Rogue Trader (1st Edition), pg. 153
- White Dwarf 98 (UK) (1988) "Chapter Approved: The Origins of the Legiones Astartes" by Rick Priestley, pg. 16