Realm of the Primes

After Optimus Prime flew into Cybertron's core and transferred the Matrix of Leadership to Primus, he died—or rather, his body died—and his spark was transported to the Realm of the Dead Primes. Among the dead were Amalgamus Prime, Alchemist Prime, Onyx Prime, Zeta Prime, Prima Prime, Micronus Prime, and Alpha Trion. As well as Vector Prime. They began teaching Optimus everything a Prime should know (Optimus is not a member of the Thirteen Primes, as he is the Fourteenth). Only the sparks of Primes can be transported to the Realm of the Primes. One day, while walking down the corridor of the Kingdom (castle) of the Primes, Optimus saw a Prime standing by the window, whom he had never seen before (this is the 13th Prime and Optimus is the 14th) Optimus does not know him and has never seen him before).

So Optimus sees this strange Prime.