The Beast is the combined form of the five Dinobots. Like them, his strength is awe inspiring, far beyond the other combiners. Unfortunately, he is also a mindless monster, whose single impulse is simply to kill, regardless of allegiance.
The Beast is composed of:
With the Autobots moments from death (some several moments past death) Grimlock laid all his cards on the table and combined with his team mates into The Beast. After killing the Decepticon combiners, and any Decepticon foolish enough to attack him, he turned on the Autobots. Optimus Prime attempted to reason with him, but to no avail. The Autobots were forced to kill him, which was much easier said than done.