Soul Pirates were selfish creatures who sought suitable specimens in order to sustain their mortality.
The First Doctor once fought them in 1900, London as they tried to capture innocents from the planet Earth using their anti-gravity beam. On their frigate, the Doctor reset the Rygerian controls in order to explode the ship. They were all killed.
The Soul Pirates were primitive-minded creatures who only wanted gain. They sought living beings to sustain themselves, denouncing them as criminals and murders. They appeared as pirates and each carried their own weapons.