The Venomous Abyss is the Midnight tier that decides your season. Full clears, single-boss kills and the two achievement runs everyone is chasing — Ahead of the Curve and Cutting Edge — are all on this shelf, priced per difficulty so you only pay for the part you actually need.
Every run is hand-played by a raid team that clears this tier weekly. Pick your difficulty, pick loot priority if you want the drops, and you get a schedule before the run instead of an open-ended wait.
Which raid service should you pick?
Normal and Heroic are the fast route to a full tier set and the AotC title. Mythic is the one that matters for rankings and for the Cutting Edge title, and it is sold per-boss as well as a full clear, so a single wall you cannot pass costs a fraction of a full carry.
If you only want the cosmetics, the raid mounts and the Venomous Abyss trophies are listed separately — no full clear required.