Those captains of the Luna Wolves (and the Sons of Horus after the Legion's name change) who were chosen to serve in the Mournival were generally considered the best officers of the Legion by their fellow Battle-Brothers. In general, each of the four members of the Mournival were chosen because they each possessed very different personalities and characteristics that were intended to complement one another and provide the Warmaster with a wide array of different viewpoints. Each of these personalities accorded with the ancient Greek Hippocratic medical theory popular during the Middle Ages of the Four Humours or substances of the human body whose relative proportions and amounts shaped all physical and mental characteristics - blood (sanguine), yellow bile (choleric), black bile (melanchol