The Adeptus Mechanicus are one of the many factions of the Imperium of Man in Warhammer 40k.
Based on the planet Mars, they're the technical and engineering forces behind the weapons and machines of the Imperium. Much knowledge was lost in the Horus Heresy, and as a result the modern Adeptus Mechanicus are almost cult-like, worshipping the machine spirit which 'lives' inside all of their creations.
They are themselves are a fusion of flesh and machine, augmenting their bodies with their technology.
Belisarius Cawl leads their order, and has achieved many things in his long lifetime, not least the creation of the Primaris Space Marines. With Cawl being allowed more free-reign since Guilliman's return, the Mechanicus have created a number of new designs to boost their own ranks. These include flying machines like the Archaeopter, and the mechanical steeds of the Serberys Raiders.