The Enigma of Combination
Creator: Primus
Owner: Formerly Nexus Prime
The Enigma of Combination was originally a gift given to Nexus Prime by his creator Primus. For years Nexus never used it, having no idea even how to use it. However, long after his tribe had began and became successful, Nexus found out how to use it, and his spark was promptly ripped from his body.
Later, when five of his pupils went to investigate his death, one of them picked up his artifact and accidentally activated it. The five combined into a gestalt and Nexus Prime was reborn. Upon further experimentation, it was revealed that the Enigma needed to take a life for a combiner to be made and reborn. This artifact would be very useful in the approaching war of the Primes.
Nexus Prime, with the Enigma in hand, would take the lives of several of his opponents troops, including Bruticus and Menasor of Megatronus's forces, Predaking of Onyx Prime's tribe, and Piranacon from Quintus's tribe, among others. However, Megatronus would go on to nearly kill Nexus, with his last act being for his spark to be put back into he Enigma of Combination, not giving his brother the satisfaction of killing him.
Since then, the Enigma has been used several times to create combiners, by Autobots and Decepticons. If there's a combiner, there's a great chance that it was formed using the Enigma of Combination.