The captured guard cannot tell the company where to find Lazaro. He knows only that the hunchback carries the key and tends something below. Lazaro eventually comes down the stairs with a basket, sees the strangers at the vault door, and tries to retreat.
They catch him. Aruma takes the single black key from his belt. Lazaro asks them to take the doctor and leave the girl alone.
The work cannot wait
He explains his duties in practical terms: fresh water, fresh candles, repair a symbol when it cracks, relight a candle when it goes out. Whatever happens to him, he wants permission to keep tending those protections.
Inside the vault, Lazaro helps the company past the guards by insisting that they want only the physician and that he must check the seals. They keep him close while he works around Estafania's chamber. He does not carry the key to Coronel's cell.
As the company prepares to leave with the physician, Lazaro sees the arrivals at the door and looks terrified. The reason for that particular fear has yet to be explained.