Netherbrain — Final Boss (3 Phases)
Location: Upper City → Avernus battlefield → Netherbrain interior → Crown of Karsus platform.
No rest between phases. Plan resources accordingly.
Pre-Fight Checklist (before entering Morphic Pool)
- Long rest — final camp rest. Dump ALL gold into consumables (elixirs, scrolls, potions).
- Heroes’ Feast (Shadowheart) — mandatory.
- Aid — upcast to 6th level for +25 max HP per character.
- Mage Armor on sorcs.
- Elixir of Vigilance on everyone.
- Elixir of Cloud Giant Strength (STR 27) on Lae’zel and Karlach if running STR builds.
- Stoneskin on Lae’zel (Shadowheart pre-cast).
- Stockpile scrolls: 3+ Scrolls of Globe of Invulnerability, 3+ Scrolls of Chain Lightning, 5+ Scrolls of Misty Step, 5+ Supreme Healing Potions.
- Call all allies — every “Gather Your Allies” option you earned. Summon them as needed (they’re spells, not pre-cast).
- Keep the astral-touched tadpole — flying and bonus-action powers in Phase 2.
Phase 1 — Avernus Courtyard (High Hall)
Environment: Ruined courtyard. Nautiloid overhead fires bombardment. Waves of Absolute cultists, mind flayers, and intellect devourers.
| Aspect | Detail |
|---|---|
| Goal | Reach the Crown of Karsus at the far end of the courtyard. Kill everything in your way. |
| Waves | 3-4 waves of enemies. Mind flayers are priority — their Mind Blast (AoE stun) can TPK if multiple hit. |
| Nautiloid | Bombardment every 3 rounds — random AoE that does NOT discriminate. Move out of red circles. |
| Allies | Summon 1-2 ally groups (Aylin, Isobel, Harpers, etc.). They distract and absorb hits. Save the rest for Phase 3. |
Tactics
- Mind flayers FIRST. Counterspell their Mind Blasts. Disarming Attack/Trip Attack. They’re squishy (AC 15, ~80 HP) but devastating if ignored.
- Use the environment. There are explosive barrels and chokepoints. Cloud of Daggers in doorways.
- Conserve resources. Don’t burn 6th-level slots. Save Twin Chain Lightning, Disintegrate, Action Surge for Phase 3. Use cantrips, 1st-3rd level spells, and weapon attacks here.
- Movement is key. Misty Step, Fly (astral tadpole), Jump — get through fast. The longer you stay, the more waves spawn.
Phase 2 — Climb to the Crown
Environment: Vertical climb inside the Netherbrain. Platforms + tentacle bridges. Nautiloid bombards from outside. Mind flayer patrols on platforms.
| Aspect | Detail |
|---|---|
| Goal | Reach the Crown portal at the top of the brain interior. |
| Timer | Implicit — Nautiloid bombardment intensifies as you climb. Move fast. |
| Mind flayers | 4-6 patrolling mind flayers + intellect devourers on platforms. Kill or bypass (Invisibility works). |
| Platforms | Tentacle bridges connect platforms. Misty Step / Fly to skip dangerous bridges. |
| Legendary Action | Mind flayers can Extract Brain on downed allies — instant death, no revive. Keep party spread out, don’t let anyone go down near a mind flayer. |
Tactics
- Fly over everything (astral tadpole). Skip as much combat as possible.
- Invisibility + Dash to bypass mind flayer patrols.
- Don’t stop to fight unless blocked. The goal is to reach Crown portal.
- Pre-cast Globe of Invulnerability just before entering the portal (Optional — risky since it only lasts 3 turns).
Phase 3 — The Netherbrain ⚫⚫⚫
This is it. The Netherbrain on top of the Crown of Karsus, surrounded by 4 tentacle mini-bosses. All previous resource conservation was for this moment.
Boss Mechanics
| Mechanic | Detail |
|---|---|
| Netherbrain HP | ~450 in Honour Mode. Resistant to all damage EXCEPT the element it’s currently vulnerable to. |
| Vulnerability rotation | Every turn, the brain becomes Vulnerable to ONE damage type and Resistant to others. Check the overhead icons. Adapt your damage output. |
| Orbs of Negation | The brain spawns orbs (platform markers). Standing on a platform nullifies the corresponding resistance. A character on the right platform → brain loses that resistance → that element hits full damage. |
| 4 Tentacles | Mini-bosses on each side of the arena. Each has ~150 HP and legendary actions. They grapple, throw, and mind-blast. Kill at least 2 before focusing the brain. |
| Legendary Actions | Mind Blast (AoE stun), Psychic Scream (AoE psychic + frightened), Tentacle Slam. |
| Absorb Intellect | Brain consumes a downed ally’s intellect — permanent INT drain that can only be restored via Long Rest. |
Kill Order
- Kill the nearest 2 Tentacles FIRST. They’ll harass your backline and throw people off platforms. Lae’zel + Bardadin each take one.
- Position on Orbs of Negation — one melee per orb, sorcs on the orbs that match their element. This strips the brain’s resistances.
- DUMP EVERYTHING on the brain once 2+ Tentacles are dead and Orbs are manned:
- Storm Sorc: Wet the brain → Twin Heightened Chain Lightning (2× 10d8 doubled vs wet).
- Markoheshkir’s free Chain Lightning (1/Short Rest).
- Wild Magic Sorc: Disintegrate (10d6+40 Force, DEX save for half).
- Lae’zel: Action Surge → 6 GWM attacks with Silver Sword.
- Bardadin: Slashing Flourish + maximum Divine Smites (4th-level slots = 5d8 each, doubled on crits if paralyzed by Hold Monster — though brain is immune to Hold Monster).
- Karlach: Enraged Throw × 2 → auto-prone the brain.
- Astarion: 5 Sharpshooter arrows with Deadeye/Arrow of Aberration Slaying.
- Summon ALL remaining allies — Aylin, Isobel, Harpers, Yurgir, Ox, Owlbear, everyone. They’re distractions for Tentacles and can man Orbs.
Globe of Invulnerability Strategy
Cast Globe of Invulnerability (via scroll) on the Orbs platform cluster. Your entire party stands inside, immune to all damage. You can fire spells OUT of the Globe. The brain can’t damage you. This trivializes the fight.
⚠️ In Honour Mode, Globe of Invulnerability still works the same way. Use it.
Honour Mode Specifics
- No reload, no retry. If you wipe on the Netherbrain, it’s over. This is why resource conservation in Phases 1-2 is critical.
- Extract Brain (mind flayers) — instant death, no revive. If anyone goes down near a mind flayer in Phase 2, they’re dead permanently (Honour Mode = no reload).
- Globe of Invulnerability is your insurance. Bring 3+ scrolls. Buy from Tolna Tome-Monger at Sorcerous Sundries.
- Ally summons are free actions — summon them on the turn you need them. Don’t pre-summon (they’ll die in Nautiloid bombardment).
- Kill Tentacles from range if possible. Their melee grapple can throw you into the void (instant death, no body to revive).
Post-Fight
- Ending cutscenes based on your choices (Orpheus vs Emperor, Gale’s orb, etc.).
- If you sided with Orpheus: he becomes a mind flayer and you destroy the brain together. Or one of your party becomes a mind flayer (Karlach volunteers; Wyll/Astarion can).
- If you sided with Emperor: he absorbs Orpheus and dominates the brain with you.
- Achievement: “Foehammer” for completing Honour Mode.