Blood Games I and II featured Vampires. They were the creatures of darkness most important to the game, and had a large amount of the text devoted to them. In OHMAS, it was Fairies - they dominated the setting in many ways, subtle and not so subtle, and also had a large chunk of text devoted to them. For Outremer, it will be Djinn.
Djinn are developed mostly from Muslim thought. Djinn are creatures with free will, like humans, and unlike angels*. They are created from smokeless fire - pure energy - as humans were created from clay. They can be adherents of any religion, and have had their own special revelations and prophets as well as learning from Human prophets.
In Christian thought, the name Genie is derived not from "Djinn" but from the Latin word Genius - a distinctive spirit who guards and inspires peoples, cultures, and individuals. Because of the similarity of sound and concept, Djinn and Genie became entwined together, and Djinn took on some aspects of the Christian Genie as Genies took on aspects of the Muslim Djinn. In Outremer, the two concepts have entirely fused.
There are four ranks of sapient Djinn - Ghuls, Djinn, Marid, and Ifrit - and two ranks of Djinn animals - yes, there are Djinn animals which exist in the same relation to Djinn as animals are to Humans. All of these can be summoned and bound. According to Islamic tradition, Solomon was a great sorcerer who bound hundreds of Djinn into servitude to him. Djinn can mate with humans and produce offspring while in human shape. The Queen of Sheba was said to be the daughter of a human and a Djinn.
This concentration on one special type of creature in each game seems to be an unintended feature of the Blood Games series. It isn't intentional, yet I couldn't conceive of writing OHMAS without Fairies, and I can't conceive of writing Outremer without Djinn. It just wouldn't work right. I wonder if this is an important part of the concept, or whether one day I will be able to design a Blood Games game without concentrating on one creature, but for now, I'm going with it.
* In Muslim Tradition, devils - called shaitan - are not fallen angels, but Djinn who have refused obedience to Allah. Iblis is their leader. Angels do not have free will, and thus could not have refused to obey God.