Hard mode Lich King is supposed to require, Blood Queen, Putricide and Sindragosa kills on heroic mode to unlock, however, this is not the case.
If you kill these 3 on normal mode, take the teleporter to the Lich King's platform, you can then set the mode to heroic, and face off against the hard mode version of the boss.
Possibly useful for people trying LK hard and not wanting to waste attempts on the other bosses since normal mode wipes do not use up attempts.
When you zone back in after the inevitable wipe, you will need to set it back to normal to get arthas to spawn, then back to heroic once on his platform.