Rakshasa

In Hindu mithology, Rakshasa, or Rakshasa Raja, is a type of demon or goblin. Rakshasas are shapeshifters and can assume the form of animals, as monsters, or as beautiful women. They are most powerful in the evening, particularly during the new moon, but they are dispelled by the rising sun. They especially detest sacrifice and prayer. First mentioned in White Night, then again in Turn Coat and Ghost Story.

Description
Rakshasas have been mentioned several times but have never made an actual appearance in the series. Nothing has been said about what they are, what they do or what they look like—yet.

Gregori Cristos distinguished himself in battle against one.

White Night
In White Night, Harry explains to Carlos Ramirez that they won't be getting much help from the White Council in their fight with the White Court regarding dead practitioners because most of the Wardens are in India battling "some big daddy Rakshasa and his forces of "old bad guys" who had been attacking monasteries friendly to the White Council while they had been preoccupied by The War with the Red Court. Harry Dresden, Carlos Ramirez, and Captain Luccio and her trainees are the only Wardens in North America.

Turn Coat
In Turn Coat, when Ebenezar McCoy told Harry that Gregori Cristos got Aleron LaFortier's spot on the Senior Council instead of Klaus the Toymaker, he mentioned that Cristos earned a name in the nasty battle against the Rakshasa that the White Council took on recently.

Ghost Story
In Ghost Story, after taking Harry to Office in Between to see Captain Jack, Ron Carmichael's ghost left explaining that he had to "get back to that Rakshasa thing."