Travel to the Bulwark - Horde territory - and assassinate Alexi Barov. Take his head and return to Weldon Barov. Be warned, Alexi will probably be well protected by Horde filth.
With the deeds recovered, there is but one thing left to do. Alexi must be killed. With him dead, I need not ever worry about an unscrupulous gang of mercenaries seeking my head.
It's a simple task. Alexi hides under the protection of the Horde - at the Bulwark.
No, I do not expect you to just walk up and kill him, I want his head! How you get it... well, that part is up to you. That type of behavior is below one of my noble upbringing.