Blackfathom Deeps

Preparation checklist

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Regroup and refill breath before each submerged stretch of the entrance approach.
If following the Horde quests, complete the crystals and Damp Note turn-ins before entering.
Assign one player to click the braziers when the group is ready.

At a glance

WoW: Forever, five players. Location: Zoram Strand in northwestern Ashenvale. Plan a first full group around 25-28 within the broad 22-32 band. Allow roughly an hour, plus the outer-cave quest objectives.

Enter the ruined temple at the strand, drop into its pool, then swim through the underwater passage and follow the naga caves to the portal. Regroup and refill breath before each submerged stretch. Horde approaches from Zoram'gar Outpost; Alliance can travel south from Auberdine. Underwater Breathing or a breathing potion helps, but the normal route has places to surface.

Current Forever sources describe a five-player dungeon with the inherited quest route and added loot, rather than Season of Discovery's level 25 raid. Tactics below use Classic evidence where a Forever change is unconfirmed. This is public-source research, not a tested clear.

Quests to pick up first

Alliance

Knowledge in the Deeps, Gerrig Bonegrip in Ironforge's Forlorn Cavern: retrieve the Lorgalis Manuscript from the underwater chest in a side alcove near Ghamoo-ra's pool. It is not a boss drop.
In Search of Thaelrid, Dawnwatcher Shaedlass in Darnassus: find Argent Guard Thaelrid in an alcove off that same early pool, then accept Blackfathom Villainy from him.
Twilight Falls, Argent Guard Manados in Darnassus: collect 10 Twilight Pendants from Twilight cultists deeper inside. Check every quest holder's count before leaving the temple.
Researching the Corruption, Gershala Nightwhisper in Auberdine: collect 8 Corrupted Brain Stems from eligible naga and satyrs inside or in the approach caves. The Stormwind breadcrumb from Argos Nightwhisper is optional.

Horde

Accept The Essence of Aku'mai at Zoram'gar Outpost first, then collect its crystals and the Damp Note in the outer caves. Return to the outpost before entering the instance: completing Essence opens Amongst the Ruins, and turning in the note opens the Lorgus kill stage. Amongst the Ruins and the Aquanis item quest require level 21.

Allegiance to the Old Gods: with Essence active or completed, loot a Damp Note from a Blackfathom Tide Priestess in the outer caves. Return it to Je'neu Sancrea at Zoram'gar Outpost and accept the follow-up to kill Lorgus Jett before zoning in. Discovering the note after the run does not give retroactive kill credit.
The Essence of Aku'mai, Je'neu Sancrea: take it at Zoram'gar Outpost and gather 20 Sapphires of Aku'Mai from crystals on the approach-cave walls. Trouble in the Deeps from Tsunaman at Sun Rock Retreat is an optional breadcrumb; you do not need a separate Stonetalon trip. Finish this collection before entering the instance.
Amongst the Ruins, Je'neu Sancrea: inspect the underwater Fathom Stone inside for the Fathom Core. A Horde quest holder summons Baron Aquanis by interacting with the stone. Kill him, then let every eligible player collect the core.
Baron Aquanis: inspect his Strange Water Globe to start this separate item quest and return it to Je'neu Sancrea. Each player needs their own globe.

Both factions and class objectives

Blackfathom Villainy starts with Argent Guard Thaelrid inside the early pool alcove. Kill Twilight Lord Kelris and loot his head, then turn in to Dawnwatcher Selgorm in Darnassus or Bashana Runetotem in Thunder Bluff. Kelris, not Aku'mai, is the quest target.

The Orb of Soran'ruk, Warlock, from level 20 in Classic: take it from Doan Karhan near Camp Taurajo and collect 3 Soran'ruk Fragments from Twilight Acolytes here. You also need one Large Soran'ruk Fragment from Shadowfang Keep.
Seeking the Kor Gem, Paladin on the inherited Alliance chain: advance The Test of Righteousness, then take the gem objective from Thundris Windweaver in Auberdine. Loot a Corrupted Kor Gem from eligible elite naga and return it to Thundris for the Purified Kor Gem needed by Jordan Stilwell. Confirm current Forever eligibility and the material stage before killing naga.
Seeking the Kor Gem, Undead Paladin, current Forever: with Trevan Rol's A Moon-Kissed Blade material stage active, accept the gem quest from Ulric Frostveil at Zoram'gar Outpost, around 12, 34.5 in Ashenvale. Loot 1 Corrupted Kor Gem from eligible naga in the approach caves outside the instance portal, then return to Ulric for the Purified Kor Gem. Take his quest before killing the naga; the Paladin guide covers the full Wolfsbane preparation.

The run

Ghamoo-ra, first large pool: clear a landing and tank the turtle there, with ranged players back from Trample. Before leaving, visit Thaelrid and the manuscript alcove, and check any Fathom Stone objective.
Lady Sarevess: clear the surrounding naga and control one guard if possible. Kill the uncontrolled guard, then interrupt her spells. Keep the tank facing her away from the party and spread ranged players to limit Forked Lightning.
Baron Aquanis, optional Horde summon: complete Amongst the Ruins while the group is together. Bring the fight to a safe place with air rather than letting the healer run out of breath while looting the stone.
Gelihast, optional murloc room: pull the room's packs back before the boss. His Net can leave a player unable to reposition, so do not begin with uncleared enemies behind the group. Inspect the altar after the kill for Blessing of Blackfathom.
Lorgus Jett, optional side objective: kill him if the group needs Allegiance to the Old Gods or his Forever-added loot. Account for Lightning Shield when healing melee, and dispel it if your group has an appropriate offensive dispel.
Old Serra'kis, optional underwater alcove: clear a route and manage breath before engaging. Water Breathing makes this detour easier. Keep the party together rather than leaving the healer on the far side of a wall or underwater bend.
Twilight Lord Kelris, temple chamber: clear the room first and begin with full mana. His Sleep can remove the tank or healer, so assign magic-dispel or backup-healing cover. Do not build the plan around interrupting every Kelris cast; the original encounter has casts that ordinary interrupts do not stop. Everyone on Blackfathom Villainy loots the head.
The four braziers: after Kelris, activate one brazier at a time. Each releases a wave of enemies. Finish the wave, recover mana and confirm the party is ready before clicking another. Lighting all four together can overwhelm a leveling party. Complete all four waves to open Aku'mai's door.
Aku'mai, final hydra: face the boss away from the group, keep ranged back and move out of Poison Cloud. Cure poison if available. Save a tank defensive, strong healing or a useful stun for Frenzied Rage rather than spending everything on the opening pull.

The early bosses and quest alcoves are detours from a direct final-boss run, but the Kelris chamber and brazier waves are part of the route to Aku'mai. Agree on skips before passing the quest areas; they are awkward to backtrack to later.

Loot worth staying for

Inherited targets to compare with current Forever items and drop tables; reported additions are labelled:

Ghamoo-ra: Tortoise Armor, Ghamoo-ra's Bind and the new Spiked Shell Band.
Lady Sarevess: Naga Battle Gloves, Darkwater Talwar and Naga Heartpiercer, whose current proc also stuns naga.
Gelihast: Reef Axe and Algae Fists; Blueprint: Fishing Rack requires 140 Fishing, giving anglers a reason to include this optional room.
Twilight Lord Kelris: Rod of the Sleepwalker and Gaze Dreamer Pants.
Old Serra'kis: Glowing Thresher Cape and Bands of Serra'kis.
Aku'mai: Strike of the Hydra, Leech Pants and Moss Cinch.
Blackfathom Villainy: Gravestone Scepter or Arctic Buckler, both worth checking before choosing a reward.
Lorgus Jett, Forever additions: Twilight Maul, Treacherous Treads and Fallenroot Longbow. Aquanis remains a useful Horde detour for his globe quest and its reward choices.

Tips

The most dangerous avoidable pull is the four-brazier event. Assign one player to click and keep everyone else away from the objects until called.
Complete approach-cave crystals and the Damp Note prerequisite before entering. Neither is solved by killing more bosses inside.
Check breath before looting underwater objects. A player who dives alone while the party moves on can be left without healing or an easy route back.
Confirm the faction-specific turn-in before hearthstoning after Kelris. The shared quest has different capital-city destinations.