The Black Disciple upgrade can be purchased at the Black Hand Nod subfaction's Secret Shrine building for 1000 credits. This is a good investment of resources even in the early game, which significantly bolsters the anti-infantry and anti-structure capabilities of the Black Hand's most basic infantry squads, and also allows them the ability to clear out buildings. In addition, it offers some extra firepower against enemy vehicles, particularly if the more expensive Purifying Flame upgrade is in effect.
The Black Disciples upgrade arguably makes the Black Hand infantry the best in the game. The already effective Confessor Cabals are almost completely unmatched by the infantry of other factions, and the normally vulnerable Militant Rocket Squads are capable of easily burning down enemy infantry, even specialized anti-infantry types such as GDI grenadiers and Scrin buzzers. They are highly effective against structures as well, including turrets such as watchtowers. They are affected by the purifying flame upgrade for even more raw power.
A strong Black Hand tactic is to rush at the beginning with Black Disciple-upgraded rocket squads and Confessor Cabals, gaining extra resources by selling all buildings except the Hands of Nod. The attack is almost impossible to deal with if not scouted since such a large force of formidable infantry will overwhelm everything in their path. One of the only possible ways of countering this, though it is still difficult to accomplish, is to get crush-capable vehicles out early and hope they will be able to run over enough infantry that they can keep them away with the support of your base defenses. However, every squad is a threat to a base defense building as every single one will have good anti-structure ability with their Black Hand squad member.
A Black Disciple trooper is identical in health and attack power to a regular Black Hand soldier. Ironically, this upgrade makes the standard Black Hand squads little more than meatshields, since both Confessor Cabal Squads and Rocket Militias can accomplish everything a squad of Black Hand troops can do whilst being cheaper and able to take more damage for their cost.