A giant spaceborne supercomputer built by the Silastic Armorfiends of Striterax. It was the first to be built like a natural brain, in that every cellular particle of it carried the pattern of the whole within it, which enabled it to think more flexibly and imaginatively, and also, it seemed, to be shocked.
The Silastic Armorfiends of Striterax were engaged in one of their numerous bloody conflicts, not enjoying it at all, decided that enough was enough, and ordered Hactar to design for them an Ultimate Weapon. "What do you mean," asked Hactar, "by Ultimate?"
To which the Silastic Armorfiends of Striterax said, "Read a bloody dictionary," and plunged back into the fray.
So Hactar designed an Ultimate Weapon. It was a very, very small bomb that was simply a junction box in hyperspace which would, when activated, connect the heart of every major sun with the heart of every other major sun simultaneously and thus turn the entire Universe into one gigantic hyperspatial supernova.
When the Silastic Armorfiends tried to use it to blow up a Strangulous Stilletan munitions dump in one of the Gamma Caves, they were extremely irritated that it didn't work, and said so.
Hactar had been extremely shocked by the whole idea. He tried to explain that he had been thinking about this Ultimate Weapon business, and had worked out that there was no conceivable consequence of not setting the bomb off that was worse than the known consequence of setting it off, and he had therefore taken the liberty of introducing a small flaw into the design of the bomb, and he hoped that everyone involved would, on sober reflection, feel that
The Silastic Armorfiends disagreed and pulverized the computer.
Later, they thought better of it, and destroyed the faulty bomb as well.
Hactar, however, survived in particle form, with enough of his abilities left to enable the supernova bomb to be built by the people of Krikkit, for the express purpose of destroying the universe. Fortunately, thanks to the supremely bad bowling skills of Arthur Dent, combined with a newfound ability to fly, the bomb was never detonated and thus, the Universe was saved.