To assemble the Trill-bot 4000 you must finish the mission, Robot Parts.
Robot Parts is an optional quest in which spawns through three Acts. It begins when the player collects the first hidden piece of a disassembled robot.
Robotic Drum: Widow's Veil (Act I)
• Location: Widow's Veil is found inside Crows' Pass (from Frost Hills or Enclave).
• Requirement: Light all the Braziers.
• The secret room is usually right behind the sitting Finnas, next to the red Portal to the Surface at the end of the instance, but can also be somewhere in the middle of the path through the dungeon.
Robotic Arm: Tower of the Moon (Act II)
• Location: Tower of the Moon is found inside Ossean Wastes.
• Requirement: Hit 4 wall-mounted brass gongs with a melee weapon or shoot them using any ranged weapons.
Robotic Pipes: Brood Hive (Act II)
• Location: Brood Hive is found inside the Swarm Point in the Salt Barrens.
• Event : Press some Switch Box's handle, Swarm Point's gate will be open.
• Requirement: Destroy all roach Eggs.
Robotic Body: Abandoned Sawmill (Act III)
• Location: Abandoned Sawmill is found inside The Blightbogs.
• Requirement: Unpin all skeletons pinned to walls and trees.
◦ Skeleton 1 is at the end of the bridge near Day One journal.
◦ Skeleton 2 is on the path to the docks and to Day Three journal.
◦ Skeleton 3 is in the second house to the left as you enter the middle of the town.
◦ Skeleton 4 is on the path out of town to Day Five journal.
• Once you find and click them, check the gate near the Day Four journal, and you should come across the Robotic Body.
Robotic Head: Cacklespit's Realm (Act III)
• Location: Cacklespit's Realm is found inside Sundered Battlefield.
• How to enter Cacklespit's Realm:
◦ Hang a Bell (quest item) found in the Blightbogs to summon Cacklespit and complete her Cacklespit's Brew quest (optional quest). The bell can be found in the Rot Gardens - it's near a post in the ruins. The place to hang the bell and summon Cacklespit can be found South East of the bell's location (Where to hang the Bell is most likely random, SE, NE of the Bell's location are most common).
◦ After completing Cacklespit's Brew quest, you must return to the Imperial Camp to get new quest Fade's Passage from Widget (middle raccoon near exit) to talk with Fade in Sundered Battlefield. You will get access to them after "Some Assembly Required" quest. His quest Cacklespit's Realm will have you collect four heads of fallen champions in that map.
◦ Upon completing Fade's quest, he will allow you into Cacklespit's Realm.
• Requirement : Destroy all (6) tree trunks in Cacklespit's Realm. After doing so, an area that was formerly a dead end, located in the lower right island - when approached - will open up a secret path to where the head is. (May have more than one location. For me it was the upper left island, near the Fade and portal to Cackelspit boss fight, the path was in the upper right part of the island.)
After all 5 parts are gathered, and Power Source is brought from the War Forge to open passage to The Broken Mines (in main questline), Professor Stoker (found in the Imperial Camp) will assemble Trill-Bot 4000 from the parts.
Trill-Bot 4000 will give you the quest Lair of the Sisters.