The Stench of Undeath

Most people don't realise just how terrible the Undead smell. The stench of rot, decay and even disease in the odd instance clings to many such creatures. Some of the worst is the zombie and the Middle Eastern Ghoul. The former is essentially a rotting corpse, while the latter is truly halfway between living and dead.
In many cases the first sign of one is the stench carried in the wind or a miasma around them, attracting carrion flies and other carrion feeders. So strong is the smell at times the fight is already lost due to this smell. Rancid meat & decaying offal drives away most living creatures & can cause gag reflexes in them too.
Many a Necromancer, Lich and Vampire uses this to their advantage to win in battle, protect their assets and to frighten off competition or heroes. A good number go as far as having such horrific creatures populate choke points and rooms where both the smell, and the numbers can be used to overrun those foolish enough to try to explore or destroy their lairs. Quite a few armies & adventuring parties have been decimated this way, and a number caught out by blundering into expertly laid traps where the nauseous smell & the undead replace a falling roof, spikes or poison gas.
Also, the smell of large concentrations of zombies & the like, directed at well-defended positions & front line troops is enough to break them at times. Many an Undead commander has used this in battle due to that trait. They sometimes use catapults to throw some zombies into the enemy too, including into beseiged towns, cities and castles.