Forbidden Underground Waterway in Bloodstained: Ritual of the Night is a sprawling river beneath the castle that connects to several areas. You’ll have to pass through it multiple times as you collect new abilities to explore the entire area.
Underground Sorcery Lab connects to Arvantville, Entrance, Secret Sorcery Lab, Inferno Cave, Hidden Desert, and Garden of Silence.
Below, we’ve included a complete map of Forbidden Underground Waterway with a few notes about the area.
Forbidden Underground Waterway map
It’ll take a couple new abilities to explore all of Forbidden Underground Waterway. After you’ve gotten Blood Steal from Bloodless in Dian Cecht Cathedral, open the path through the fountain from Entrance. Farm any Deeseama you find until you pick up the Aqua Stream Directional Shard. This will allow you to explore underwater.
Use your new Aqua Stream ability to head to Hidden Desert. There, you’ll battle Alfred and pick up the Deep Sinker Shard ability. That Shard will make you sink underwater, which lets you open all of those underwater chests.
After that, you can continue on to Secret Sorcery Lab and Inferno Cave. In Inferno Cave, you’ll pick up the Invert Shard ability — which is arguably the most useful in the game.