Rakshasa are native fiends, the children of Khyber, who appear to be humanoid tigers. They have the head of a tiger, their entire body is covered in fur, and they have claws on their hands and feet. They also have reversed hands, the palms of their hands are where the back of the hand would be on a human. Most rakshasa have the colouration of traditional tigers, but not all of them.
Much like their celestial counterparts, the Couatl, Rakshasa are shapechangers and telepaths. They can take on other forms to suit their needs, most commonly humanoid forms to better blend into society. They can all read minds, further increasing their ability to hide amongst the humanoid populations of Eberron. Unlike the Couatl, however, they cannot project their thoughts or manipulate the thoughts of others.
The most peculiar thing about the Rakshasa, though, is their innate limited immunity to magic. All rakshasa have this trait, though it varies in strength, that means they cannot be affected by spells unless they want to be. This makes detecting them extremely difficult, as most traditional forms of magical divination fail on them. They are unaffected by Zone of Truth, Moonbeam, True Seeing and other spells that would reveal them as what they are.
During the first age of the world, the Rakshasa ruled under the Na-Vakhti, the Overlords. The world was a hellish place, where reality was mutable and all were victims of the whims of the overlords. This was the case until about 150,000 years ago, when the dragons and couatl jointly discovered the Draconic Prophecy, giving them the tools to fight back against the demons.
When the couatl sacrificed most of their species to form the Silver Flame, the overlords and most of the fiends were imprisoned within Khyber. Rakshasa have ever been tricky, though, and many of them were not entrapped by the flame and remained within the world. Most of them retreated to the Demon Wastes initially, but here they schemed and hatched the millennia long plans to release their imprisoned masters.
It is possible that the rakshasa have infiltrated many of the organisations in Eberron. They are immortal spirits of deception, with the patience to play lifetime long waiting games in pursuit of their goals, and they are uniquely suited to the role of infiltrator. Their shape changing abilities combined with their ability to read minds and avoid most forms of magical detection mean that anyone you know could be a rakshasa, and it's unlikely you'd ever find out.
Types of Rakshasa
Sorcerers
The standard rakshasa, which most of this article refers to, are the "sorcerer rakshasa", just called Rakshasa. They are not the only kind of rakshasa in the world, but they are the most numerous.
Zakya
Unlike the sorcerers, the Zakya are suited to physical combat and relish in it. They are masters of combat, having had thousands of years to perfect their technique. They are less magically capable than their sorcerer kin, however, and their immunity to magic is not as potent.