The Soft Weapon

Nessus, a cowardly two-headed Puppeteer, recruits a human crew for what looks like a routine survey and instead leads them to a Slaver stasis box a billion years old, and inside it a shapeshifting weapon that can become almost anything its user needs. When the ruthless Kzin pirate Chuft-Captain seizes the ship to claim the weapon for his empire, holding onto it becomes a matter of survival, not profit.