The Netherbrain/combat

The Unbreakable Will of the Netherbrain
Huge Aberration
Level 20 Elder Brain
Stats
Armour Class	15
Hit points	300 (Balanced) 450 (Tactician)
Movement speed	0 m (0 ft)
Weight	10000 kg (20000 lb)
Initiative	-15
Ability scores
STR DEX CON INT WIS CHA 14 (+2) 10 (0) 22 (+6) 28 (+9) 21 (+5) 24 (+7)
Proficiency bonus	+4
Saving throws	Str +2 Dex 0 Con +10 Int +13 Wis +9 Cha +11
Conditions
Chains of Karsus	Will break free of its domination within 5 turns
Against All Odds	Begins with 20% of its HP missing, depending on an earlier ability check
Features
Psionic Retribution	Use a reaction to release a brainquake

Overview

Combat

This page focuses on The Netherbrain's behavior during turn-based combat gameplay.

Attacks and abilities

 Retributive Brainquake ()

10d6 (10~60) Psychic damage

Upon first time that you take damage in a turn, emit a wave of psionic force that  Mindbreaks nearby enemies.

INT Save

 Range: 30 m / 90 ft

 Orb of Negation ()

The Grand Design cannot be disrupted - manifest an orb of Netherese arcana that explodes on its turn, obliterating everything within range.

 Range: 30 m /  ft

Honour Mode

 Aegis of the Absolute

The Netherbrain can make itself Immune to all types of damage it took in a round of combat until the start of its next turn.

 Range: 18 m / 60 ft

Conditions

•  Chains of Karsus - After using Karsus' Compulsion on the Netherbrain
•  Against All Odds - If a natural 20 was rolled on the final check to dominate the Netherbrain at the Morphic Pool it will have this condition, which reduces its HP by 20%.

Allies

• Dominated Red Dragon
• 4x Illithid Arcanist
• 16x Tentacle (hidden under the brain's surface until approached)
• Nautiloid (Creature) (appears after a 4 turn countdown and begins summoning Mind Flayers and Intellect Devourers)

Conditional Allies

• The Emperor (if the player chose to free Orpheus)
• 4x Dream Guardian (if the player chose to free Orpheus)
• 4x Countermeasure (if the player chose to trust the Emperor)

Encounter

Atop the brain

• The objective of the first phase is to make it to the Crown of Karsus and channel Karsus' Compulsion on it to enter the Netherbrain's psyche and defeat it for good.
• Stepping directly on the brain's surface will summon up to 16 hidden tentacles, so it may be easier to approach from the two platforms on either side.
• Once the party has reached the Crown of Karsus, the party member with Karsus' Compulsion must channel it in order to enter the final stage of the fight. The channelling lasts for 2 turns (the turn it was started and the following turn) and on the 3rd turn a portal will open to the Netherbrain's psyche (or The Unbreakable Will of the Netherbrain).
• Only party members and either Orpheus or the Emperor can enter this portal, so once it has been opened there is no reason to continue fighting other enemies.
• Once all party members and Orpheus/the Emperor have entered the portal, the battle outside on the surface of the brain will end and the remaining turn order will just consist of the party and the Netherbrain.

Inside the crown

• It is strongly recommended to have the entire party near the portal before completing Karsus' Compulsion. Once the portal is opened, the final fight against the Netherbrain begins immediately and it must be defeated before it destroys all the platforms inside.
• While allies summoned through Gather Your Allies (Class Action) cannot enter the portal, they can be summoned onto the platforms inside.
• The Netherbrain will use  Orb of Negation to summon several orbs which deal damage at the end of the next round and destroy the platforms beneath them, so it is important to move characters off of any marked platforms. These Orbs will disappear upon the brain's death.
• The Netherbrain must be killed quickly since every platform will be destroyed in only a couple of turns.

Tactics

Atop the brain

•  Dimension Door,  Arcane Gate, and  Misty Step are all useful spells/scrolls for moving from the entrance Atop the Netherbrain to the Crown of Karsus. Otherwise, if intending to sprint past the tentacles hidden under the brain's surface, it is helpful to use a character who can avoid Opportunity Attacks, such as one with the Mobile feat.
•  Globe of Invulnerability and  Sanctuary are useful for protecting the caster of Karsus' Compulsion. However, the nearby Illithid Arcanists will still be able to  stun creatures affected by these spells, which disrupts the spell prematurely.
• The four Illithid Arcanists are very dangerous enemies in this section of the fight. They can spam a more powerful version of  Magic Missile, use  Counterspell, stun your party via  Tentacle Whip and  Mind Blast, and even gain control of allies with  Mind Flayer Domination. They are also immune to being pushed over the edge with moves like Telekinesis, as they will just reappear atop the Netherbrain. Fortunately they have a rather low number of hit points, making them vulnerable to allies with high maneuverability and damage.
• While choosing to ally with the Emperor over Orpheus results in less total enemies during this fight, the Countermeasures are far more dangerous than the Dream Guardians. Some of them have powerful disruptive abilities such as  Counterspell,  Psionic Dominance,  Goading Attack (Melee),  Slow,  Black Hole, and  Confusion which can severely hinder an unprepared party. Others have significantly more hit points (see Rage Queller) or deal far more damage through  Improved Extra Attack and  Action Surge (See Breaker of War).

Inside the crown

• Casting  Feather Fall before entering the portal can allow melee range party members with low movement speed to immediately jump down to the brain and begin attacking it.
• The first attack each round against the Brain will cause it to use  Retributive Brainquake, which will damage and possibly  Slow everything in range. The player can potentially avoid this the first time by triggering it with the first party member to enter the portal, and then having the rest of the team enter afterword.
•  Dimension Door and  Misty Step can help party members reach safe platforms if the ones they are on have been designated for destruction.
• On Honour Mode, the Netherbrain will augment itself with  Aegis of the Absolute, making it immune to damage types that were used agains it on the previous turn. It is important to plan ahead exactly which attacks will be used against it so that the party doesn't miss entire turns where the brain is immune to all their attacks.

Achievements

Hero of the Forgotten Realms
Save the day: kill the Netherbrain and destroy the Absolute tadpoles.

Absolute Power Corrupts
Reign with terror: take control of the Netherbrain and bend the world to your will.

Sins of the Father
Claim your Throne of Blood: take control of the Netherbrain for Bhaal and break the world.

Known Bugs

• Thrown weapons that return to the user, such as Nyrulna or Returning Pike, may occasionally fall into the chasm below the Netherbrain and disappear forever after being thrown at it. Throwing builds should bring back-up weapons just in case.