Agga is a ghost that appears when you try to enter the Thieves' Passage. He will ask you a riddle and only the correct answer will grant you passage.
If you give up and walk out the door, Caleb will begin talking and saying he realized the answer is 'coffin'.
"Soundless it flies. It's faster than me. In darkness it dies. It's all we can see.
What is it?"
Light. (Answer in older versions - V 2.5 or lower - was 'Fire')