The Motherboard doesn't spawn on it's own. The only way to summon it is to use a Mechanical Brain. It can be summoned anywhere at night.
Behavior
The Motherboard can fly, shoots beams similar to those of the Necromancer, and disappears similarly to the Brain of Cthulhu.
Attacks
Phase 1
The Motherboard is invulnerable during this phase. The only way to deal damage to it is to kill all of their Signal Drones which puts them into their second phase.
The Motherboard will constantly spawn in new Signal Drones to replace destroyed ones. Try to kill all the drones at once to defeat the Motherboard easily.
As the Signal Drones only method of attack is to connect with other Signal Drones via a purple beam, they only pose a threat to Melee players.
The Motherboard occasionally shoots 3 Necromancer beams at the player.
You must get it to Phase 2 as soon as possible; the beam is extremely accurate and undodgeable without a Rod of Discord.
Phase 2
Once all of the Signal drones have been destroyed, the Motherboard will enter its second phase.
4 Clampers will be spawned as tendrils from the Motherboard, but they don't attack aggressively and only follow the Motherboard.
Clampers deal damage even from their wire so try to stay above the Motherboard to reduce damage taken.
Once all of the Clampers have been destroyed, The Mothership will deploy 2 loose Clampers with significantly reduced health, which will fly at players like demon eyes.
It will now teleport randomly near the players similar to the Brain of Cthulhu.
You have until dawn to defeat it. If not killed when daytime arrives, the Motherboard will enter into rage mode and receive an insane speed buff and instantly kill any player it comes into contact with.
Trivia
The Motherboard Boss and attacks came from suggestions made by Zoomo
A current bug with the Clampers is they may have increased health from designated totals. EX: 6430/2400.
There is a bug where The Motherboard cannot be spawned in multiplayer.