“The 5-year Anniversary Onyxia encounter will have no Heroic mode”—according to Zarhym. It will only have the 10- and 25-player normal mode versions. Both modes will have an equal chance to drop a 310% speed Brood of Onyxia mount.
There is no lore behind the return of Onyxia, nor expect any. This is simply a reward to players to celebrate the 5-year Anniversary of World of Warcraft. There have been similar special rewards before granted by Blizzard to celebrate other meaningful dates.
Read the recent announcement of the 5-Anniversary Onyxia Dungeon Overhaul.