Brain of the Many

The Brain of the Many is its central mind and the first boss in System Shock 2. Soldier first sees the Brain in a hallucinatory sequence on the Engineering Deck, and finally fights it in the Body of the Many.

Physiology
The Brain is most likely the cognitive center of the Many, however the specifics of its functions are unknown and unexplained. It displays a degree of intelligence and is aware of player's actions throughout the game, thanks to the psychic link with the Annelids. The Brain also possesses great Psionic power, which it uses to both protect itself and communicate directly with the player.

Personality
The Many view individuality as a weakness and a source of conflict. They also believe that they must bring an end to the human discord by absorbing all life. The Many are not above using violence to ensure the success of their goal - aside from the invasive conversion process that turns humans into shambling Hybrids, they are also responsible for most of the atrocities aboard the Von Braun, including the creation of the Cyborg Midwives.

The Many display some hypocrisy: they rail against cybernetics and machinery, yet it was them who instructed Marc Miller how to create the Midwives. They hate chaos and discord, yet they incited much death and suffering directly. Most likely the Many perceive their actions as being in the interest of what they consider the greater good.

The Many are cordial and measured in their communication, never insulting soldier directly. Even when angered, they leave the aggression to their minions.

Strategy
The Brain itself does not attack, but is almost invulnerable until its three Psionic Defensive Projections (also called Brain Defense Nodes) are destroyed. The Projections (previously unseen in the cutscene) orbit around the organ and protect it, lighting up when the Brain is attacked. The Brain Chamber and its entrance are guarded by numerous Rumblers and Psi Reavers. There is also a unique, powerful Greater Psi Reaver floating above.

The Psi Reavers' brain structures are located in the side niches of the Brain Chamber (the Greater Psi Reaver's brain stem is located in the middle recess on the left side). Photonic Redirection Psi Power (preferably combined with Localized Pyrokinesis cast before) is more than recommended to deal with them (and a single stalking Arachnid). Afterwards, the physical forms of the Reavers can be destroyed for good.

The Rumblers pose a significantly greater threat in the long run. When killed, they respawn in nearby vicinity almost instantly... and also infinitely! The player must stay on the move all the time to avoid being cornered and ripped apart. Up to four Rumblers can follow the player. A good method is to lure them all into a secluded area (the corridor connecting the Brain Chamber and the organic pool) and trap using Metacreative Barriers. These last for 4 minutes, which should be enough time to deal with the Brain leisurely. An easier approach is to cast a single Psi Wall and climb atop of it. The Rumblers won't be able to hit you. Alternatively, Imposed Neural Restructuring can turn their relentless brute strength against other enemies. It is possible that a Cyborg Midwife will occasionally appear instead of a Rumbler.

The floating Psionic Defensive Projections have just 10 Hit Points each, so a few shots with even a Pistol will suffice. They are also susceptible to the splash damage of most Heavy and all ranged Exotic Weapons. On the other hand, these Defense Nodes are notably harder to hit with melee weapons.

As for the Brain itself, any weapon (beside SFD) will be enough to destroy it. The Projections and the Brain are most vulnerable to Anti-Annelid attacks of ranged Exotic Weapons, however both Anti-Personnel ammunition and Incendiary attacks are also very effective against them. The Brain explodes upon death, so be careful. While the explosion deals no damage, it can launch the player quite far, thus keeping the distance is recommended.

Alternatively - with Soma Transference - it is possible to target the Brain directly (without wasting the Projections) to drain all its health away. This can save from fighting all of the creatures surrounding the Brain. It is highly recommended to max the Power by Overloading, as it greatly influences the amount of transferred Hit Points.