A term popularized by the Cthulhu Mythos; the word "eldritch" means something like "coming from a foreign realm". These beings are the strangest the universe has to offer; even for alien standards they're otherworldly. They are usually extremely powerful and typically display unexplainable and supernatural abilities.
They may manifest themselves either corporeally or noncorporeally. In some cases, their physical manifestations constitute a mere shadow of what they truly are, and they may have several avatars.