The Missing Brother is a side quest in Egypt (Act II). It is given by a woman named Tathari, who's standing near the Portal in Memphis. She tells you of her brother Unas, who wandered off to their family tomb to wake an ancestral guardian creature. Tathari requests you to look after her brother and bring him back.
You will find Unas in a tomb, which lies to the east of the Giza Plateau. He will tell you, that he managed to wake the creature, but it turned against him. The creature is a hero Tomb Guardian called Hesy-su-neb-ef and it's waiting behind the large door next to Unas. Hesy-su-neb-ef can be fairly dangerous, but the door will stay open and there are no other monsters inside the chamber.
After you have killed the creature, you may talk to Unas, but he will just say thank you. Return to Tathari in Memphis to get a reward and to complete the quest.
Reward: 10000 Experience and a random magical ring.