Tomb Guard are the Undead guardians of the Tomb Kings. In life, the Tomb Guard were faithful bodyguards, or favoured warriors who served their liege well, and were rewarded with partial mummification - though by no means as ornate as that performed on the Tomb Kings and princes.
When Nagash performed his Great Ritual, and brought the inhabitants of Nehekhara back to an unholy mockery of life, the ritual mummification the Tomb Guard recieved allowed them to retain some of their living identity (in a similar manner to the Tomb Kings), and allowed their spirit to retain some awareness, in contrast to the rank-and-file skeletons who are merely animated bones. The Tomb Guard are much more vicious warriors than regular skeletons as a result of this, weilding wickedly curved blades and wearing ornate armour. Unlike the sighless eyes of ordinary skeleton warriors, a malign awareness burns within the dilapidated body of the Tomb Guard, making them much more dangerous opponents. Thus the Tomb Guard continue to serve their lords, standing a silent, skeletal vigil against those who would defile the resting places of those they served in life.