Story achievement, can't be missed. Quest starts at Elea VII, is called Unidentified.
Next go to Elysia Port and Elysia Lambda, where you'll fight two pirate ships. Then back to Elysia Port.
Next stop, Polaris system, New Ethiopia. You'll fight an unknown vessel. Once you beat that, you'll get the achievement.
So what does this have to do with capturing a soulless? You'll find out as you play along.