What happen with old Frostmourne?

Sry guys if anybody can inform me I couldn't find proper information about old Frostmourne realm, to be more precise I mean about the characters inside. I didnt play realm 3-4 mounts and I wanna log on back. I realized that my chars are lost. Then I saw into the acc services that there is an option to retrieve them , but only to transfer them to Icerown realm. My point why is that, is the realm ended with content? What happen ? Is the Frostmourne started again from the beginning

Sry I saw when I logged inside the the realm there is a script on the screen. LOL it said that is ended and we can migrate......sry guys for losing your time. See ya inside the game !!!