How to Make Lord Harkon Your Thrall and Why Not to Do So

Here is a link to Sickliff's video on Harkon:    • Make Skyrim BEATABLE Again (with vamp...  
It showcases how to access the Harkon dummy and clones early in game.

There are a few ways of reanimating Lord Harkon to use as a follower. You can use the Ritual Stone combined with the Aetherial Crown, to reanimate him as you please. It always works on a maxed Harkon, since he caps at level 60. Up to level 12, he can be reanimated using the Staff of Dread Zombies, as it will not disintegrate NPCs.

The two Harkon clones are located in the final battle room with Harkon, and are used in the battle. It is a different cell than the one that can be accessed right away if siding with the vampires in Dawnguard. The door that opens this cell is triggered during Kindred Judgement. However, it can be accessed early via load warping, allowing access two both the Harkon clones. However, killing the magic user one will break the questline, as his copy is considered to be the real Harkon during the final battle.

The song playing is the theme from Luigi's Mansion.