Arguably the most sophisticated warjack ever built by human hands, the Thunderhead is a marvel of technological innovation and sheer artistry. This powerhouse of a warjack, a source of awe and devastation on the battlefield, relies on technologies developed by Sebastian Nemo decades ahead of their time. It draws its power entirely from galvanic energy generated by its massive storm chamber. Nemo originally envisioned a warjack fueled by arcane electricity, able to function without coal or water or the telltale signs of steam power, but he never found a way to disguise the brilliant corona of energy surrounding the massive construct. The air around the mechanikal behemoth hums with pale, crackling arcs of electricity as its galvanic aura scorches the ground with each step. The Thunderhead is no subtle or quiet machine—but it does not need to be.
After several prototypes that failed in spectacular explosions, Nemo finalized construction plans in 604 AR. The size and complexity of the machine required every part to be carefully fabricated to the senior warcaster’s exacting specifications. Its design pushes the envelope of modern mechanikal science and integrates a number of technological advances initially developed by the Cult of Cyriss and reverse engineered by Nemo. Though thousands of crowns’ worth of equipment were destroyed and two assistants were badly burned developing its power generator, Nemo proceeded with the project until he knew he had a workable device that could generate sufficient power and still remain stable enough to be viable in a heavy warjack chassis. The result was a new pinnacle of warjack design.
The Thunderhead is large even for a heavy warjack, its great lightning coils rising from the rear of the machine in place of conventional smokestacks. Its carapace is composed of huge, graceful plates of curving steel that bulge as if barely containing the energy within. A blue-tinged glow seeps from the joints and edges of the armor. The twin brass-capped coils tower a full five feet over the hull, and arcs of electricity continually lash the air between them. So powerful are the galvanic forces involved that it took Nemo over six months to engineer shielding to protect the ’jack’s own cortex. All the Thunderhead’s weapon systems were designed to take advantage of its galvanic heart. A pair of enormous lightning coils channel the electricity generated by the oversized storm chamber into devastating pulses, annihilating anyone who comes too near without the protection of mechanikal insulation. Alternatively, the pulse can be narrowed into directed blasts through the smaller arrays of coils that run down the warjack’s massive fists. In melee combat, lightning plays across the Thunderhead’s fists and grounds through anything smashed by its powerful blows. These energies sometimes surge unpredictably, frying the cortexes of enemy warjacks. 
Given its staggering cost and technical complexities, this is not a machine Cygnar can afford to manufacture in great numbers. As a result, the Thunderhead is reserved for only the direst of battles where its presence can assure crucial victories for the crown.