I would say very good army, definatly dont replace the fiends as they are disgusting when they get incombat which is very quickly!!! Maybe drop a unit of plaguebearers for bloodletters as horrors also make quite good icon bearers!
Oh and get a changeling in a pink horror unit!!!!! Its the main reason for having a horror unit and at 5 points you cant afford to not have it.
Also dont bother with pavane of slaanesh on the keeper of secrets as it means he cant fleet of foot when it is used. Give him the Hit and run ability soporic musk or somthing like that.