Beings of Tash Kalar
You can see Bull's head, with two glowing eyes.
Once unleashed, the bull runs through everything. Often, it includes pieces that are part of his own pattern. If the bull kills a legend, it calms down a bit and stop.
Of course, Hell Bull can kill a legend. Or furrow a long gap in the opponent's patterns. But he is also great for fulfilling tasks that require upgraded or legendary pieces at colored spaces, as only few legends can move after summoning.
Beware of downsides. A lone bull surrounded by enemies can turn against you. Unless you plan to summon a Cavalry Captain for more destruction, of course.
Hell Bull is one of the original six legends, but no legend probably changed its pattern so many times during development. The effect was always similar (the bull gets unleashed and runs), but the details varied - whether and how much legends it can kill, whether he can stop or has to run until is stopped etc.
Have you ever fulfill Envelopment or Isolation tasks by trapping a Hell Bull that recklessly ran into your ranks?