The Midnight Rakghoul is a hidden ‘Bonus’ Boss in the Kaon Under Siege flashpoint in both Veteran and Master Mode. It is considered a bonus in the fact that the Midnight Rakghoul is located away from the main path within the flashpoint, and hidden due to the fact that it only spawns during an ongoing Rakghoul Event week in SWTOR.

The Boss essentially has one special move where it explodes in a cloud of green-mist that removes any active Rakghoul Vaccines on your character, so taking a Rakghoul Relic or spare Vaccines is a must if attempting this boss to avoid getting the plague on your character.
You can find the Midnight Rakghoul just before entering the final Spaceport map of the flashpoint. In the South-West corner of the Transit District map, head right when walking up the staircase to the Spaceport entrance and loop around West above ground to the upper-highway platform, there you will find the Boss running around a security fence.