It's not about something so simple as a girl, it's about a deity. One called "Sleep" that visited Vessel in a dream and promised him glory in exchange for worship and servitude. So Sleep Token lives to spread the gospel of Sleep. Vessel is a title that was appointed to him when he agreed to devote himself to Sleep. It's the biblical use of the word "vessel". To mean one through whom a greater being acts.

The trilogy of albums details Vessel's journey with Sleep which does strongly parallel a romantic relationship (and is one on many levels), particularly one with with an unbalanced power dynamic that Sleep completely leverages against Vessel. Sundowning is basically about the beginning of their relationship with Sleep effectively grooming Vessel or at least luring him in. This Place Will Become Your Tomb features really prominent imagery of how he is trapped in this relationship with Sleep and the damage that it's causing him.