Just before you encounter the altar, you'll see a message on the wall stating that you should say the missing brother sent you. The brothers you'll encounter in the game are Alpha, Beta, Gamma, Delta and Zeta. If you're familiar with the ancient Greek alphabet, you'll recognize these names as letters in said alphabet. Then you might also know that there's another letter between Delta and Zeta... EPSILON. That is the missing brother, so answer the altar with that word. I really have no idea how you're supposed to find this out without knowledge of the Greek alphabet. Maybe those New World Computing guys forgot something?