The missing brother

Just before you reach the altar, a message on the wall says you should tell it the missing brother sent you.

The brothers you meet in the game are Alpha, Beta, Gamma, Delta and Zeta. If you're familiar with the ancient Greek alphabet, you'll recognize these as letters in that alphabet — and you might also know there's another letter between Delta and Zeta: EPSILON. That's the missing brother, so answer the altar with that word.

How you're supposed to figure this out without knowledge of the Greek alphabet is anyone's guess. Maybe someone at New World Computing forgot something.

Related area: Slithercult Stronghold