Immortals are a race of beings that exist outside the Cycle of Life and Death. They follow the Will of the Galaxy Universe Dark, with the wish to end the cycle so that the Universe may never fall. 

Immortals VS Vampires

While alike, Vamipres are in fact more like the Earth-cousin to the Immortal race

Immortal bodies

During the series, it can be seen that Immortals can have no physical form. Dainn had none, instead stealing over the body of Ringo. Through this, this is an example of actual fusion in the series