First of all I really like the witch-king skill “Might of witch-king”.
It had once been an ability of the fortress after the player used the spell. But that had been before this spell got so many extra abilities. I do think that it wouldn't be good to move it back in there, since the fortress already is really good and has so many abilities, that this really complex one would be too much in my opinion. On the other hand, I think that it could be really nice as a central spell, since the central spell right now isn't that good at the beginning of the game, when you have one or 2 heroes, but gets really strong when you got all of your heroes out and leveled them up. Although I think that the central spell should be good at any time of the game.
I really like the idea that the Witchking doesn't have to be on the battlefield to give power to one of his servants. In fact, all the heroes of Angmar who are not connected by friendship but only by their lust for power, I think it fits more that the Witchking declares one of the heroes as the leader of them all while he is not present at the battlefield. Positive side effect: Angmar players would focus more on the heroes he got instead of just spamming all of them out.
I'm not quite sure if it could be a bit too strong for a central spell, since you could get a really fast Drauglin-Werewolf. I'm not sure if that would be so balanced but since it costs nearly as much as the turncoats it also can be fine or could be fixed by raising Drauglins revive price after he was a Werewolf.
So I'm
against putting the spell into the Fortress of the Iron Crown but
FOR using it as a central spell.
Here is my suggestion of how this new central spell could work:
Might of the iron crown: The Witchking declares one of Angmars heroes as his deputy. When used on a hero, the chosen hero gets a strong bonus effect (as usual), while other heroes of Angmar get -10% armor for that time.
As the Witchking reaches the battlefield, all heroes abilities recharge X% faster.
Hope I described my thoughts in a comprehensible way