Cromwell is a legendary sword forged in a time long before the rule of Princess Celestia. It is named after the spirit of the legendary warrior that resides inside it.
Sword of Cromwell was once wielded by it's namesake, the legendary warrior Cromwell, who fought against the dark forces overtaking the world before Celestia came to rule. When he died, he had his spirit infused into his sword, and became the soul residing inside the blade. Due to unknown circumstances, the blade was eventually lost to time, until it was recovered thousands of years later by Adam.
Cromwell is a very exceptional blade, being nearly indestructible due to the soul residing inside it. The sword has the ability to make multiple copies of itself to assist in close range combat. The spirit of Cromwell is able to communicate with the weilder via a telepathic bond, and can provide helpful advice due to his wise age and battle experience. Cromwell is only usable by select people, being the person either the wielder or Cromwell chooses. The blade is naturally extremely powerful, able to cause massive damage and is capable of powerful magical abilities.