I stumbled across this pairing completely by accident, honestly. It was in a huge multi-chapter fic focused on another ship, and their scenes together just hooked me. The writer was playing with this idea of mirrored damage—both characters are these powerhouses who have been through horrific things, but Abel's trauma is worn on the outside, in his physical state, while the Abyss's is all internalized, this deep, cosmic loneliness.
What makes the emotional tension crackle, at least in the best fics I've read, isn't just them comforting each other. It's the friction. Abel, with his human(ish) history and guilt, trying to apply logic to something fundamentally alien and ancient. The Abyss, in turn, perceives his suffering as something beautiful and profound, which is both unsettling and weirdly tender. That gap in understanding, where one character's pain is the other's aesthetic, creates this delicious, awful ache. You're never sure if it's healing or a deeper kind of corruption, and that ambiguity is the whole point.
A lot of it hinges on the Abyss being an entity, not a person. The best writers don't try to humanize it too much; they let it be vast and strange, so Abel's attempts to connect feel even more fragile and brave.