The reign of Christ and His priesthood comes to an end. When the Sonship is brought under subjection to God and God becomes all-in-all there will no longer be a need for the mediatorial role of Jesus Christ. This will be when all the purpose and plan of God for all the ages comes to fruition. Everything that God has been working for is suddenly realized right here. We do not know what that looks like!
The concept of eternity is difficult to grasp. For instance, is it time without an end or the complete absence of time? Either way, it is an abstract idea that we have been wrestling with since Plato first introduced the concept. Yes, Plato the Greek philosopher, introduced the idea of “forever”.