From the website:
The Black Coaches are funerary carriages from the underworlds. Within these arcane carriages any tangible remains are gathered in a casket that offers a safe haven in which the banished entity can reform. The dormant occupant can feed upon the energies of death, drinking in agony and grief in order to grow strong. On first tasting death, the Black Coach speeds forward before shimmering jets of balefire extend from the wheels, acting like scythes to cut down enemies. When the coach is immersed in sufficient energies it flickers, allowing it to disappear and reappear in more advantageous positions. Onwards it rolls, growing ever more powerful until it becomes a nigh-unstoppable force of death.