Out of the Water
Follow the tunnel beyond the Serpent Hole as it curves around the edge of a large flooded cave. Dive into the water, swim for the Demonic Artifact near the Floating Mine spawner to the right, then make for the red-lit tunnel to the east.
This tunnel leads to a flooded cave loaded with Floating Mines and various rock spires that can make dodging them tricky. Move fast—don’t forget that you can dash underwater—and pluck the Angelic Artifact from the seafloor. Swim to the east, surface quickly, and leap onto dry ground before the Floating Mines detonate.
The ruined train station ahead is filled with cocoons, many of which contain a Blue Mimic ready to attack. Break them open one at a time to clear the room of their threat, then pay Vulgrim a visit in the corner.