Back outside, you’ve two possible paths ahead. Take the south-eastern route down to the water and bear right. There’s a chest in the small alcove to the south containing a Dark Chainmail Helm. You’ll find a second chest, containing a Sharpening Augmentation, on top of the island in the middle of the water. Unfortunately, there’s an annoyingly persistent Nymph to deal with here and, as a final insult, a huge swath of Undead opponents emerge from the water once you reach the island’s peak.
Make your way back to the point where you first entered the area, forging a safe path through the Undead, then take the north-eastern path. Eventually, you should spot a small hill to the north leading to another chest. There are a couple of Undead Generals who just love to get in the way here, so deal with them first then raid the chest for a Lightning Augmentation. Finally, dig in the mushroom ring to the right for a Silver Key.
With all that out of the way, head into the domed vestibule to the south-east. Before nipping through the door, though, look right and you’ll see another chest containing a Resurrection Phial. Once it’s in your possession, swing open the door and—oh, wait, no. Once again, all exits are suddenly, magically sealed, leaving you to deal with a surprise Undead ambush. These, alas, are slightly stronger variants than before, meaning it’s trickier to break through their defences. Once they’re down, though, head south-east into the Underground Passage.