In the Citadel you will get a quest from Dr Tidemann to visit Darwin Village to find a cure for cancer. Darwin Village is a higher level area with enemies that have a lot of HP and are significant in number. Fortunately they are slow are very easy to hit.
Once inside Darwin Village you will find an entrance to an underground complex.
Note: Having Rose in your party allows you instant access to the complex by simply "using" the console leading to the complex. Unfortunately if Rose IS in your party you cannot earn the achievement Sinister Legacy.
When you enter the second portion of the underground complex there is a locked door (Computer Science check) on the same wall as the ladder you came down. Heading through this door will lead to a containment area that is guarded by 4 sentry turrets.
Note: The corridor leading to the containment area is full of landmines. Either send in your solo character that has a high perception. Or send a solo tank and trigger the mines so only he is hit.
Upon clearing the area you will need to unlock the containment cell which again is another high computer science check. After that a delightful little ... errr ... guy? tags along with you. He doesnt do anything in combat apart from rattle off very friendly one liners.
Anyway. This guy, is the Night Terror. To unlock the achievement from this point you need to feed him 20 pieces of "Candy".
And Candy to the night terror is organic junk. This includes human hands, cow ears, pickle jar, cow droppins, eyeballs etc... Nice right ?
Leading up to getting the Night Terror you can bring along 20 pieces of "Candy" with you, or you can feed him as you loot it. I personally went back to the Rail Nomads camp and bought back a lot of the junk I sold to merchants and fed him in one hit.
Hope this helps.