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The death-knights recognise you whatever appearance you take. The human guards won't recognise you though.
The best way to get past the death-knights is to watch their patrol ducking into rooms and around corners where they don't go. When they pass your hiding place carry on in the opposite direction. Eventually you may be tough enough to fight them (especially if you gain experience in the Battlefields) but at first you'll need to avoid them.
Your death-knight companaion should tell you when you are entering/exiting the patrolled areas, which also helps.