Travel to Chillwind Camp - Alliance territory - and assassinate Weldon Barov. Take his head and return to Alexi Barov. Be warned, Weldon will probably be well protected by Alliance scum.
Naturally, with my brother still alive, there may be some contention as to the primary beneficiary of the family's fortune. This cannot be.
A low down guttersnipe like you isn't above a little cold-blooded murder, I hope.
It's a simple task and one that I expect you will complete; that is, if you wish to claim your reward.
Weldon sits under Alliance protection at Chillwind Camp, to the east. Slay him, return his head.
Hurry along now.