TLDR
Hijack organic enemies in the most visceral, horrifyingly satisfactory manner possible.What?
The brain harpoon would be unlocked on Gleba, presumably at the end of the local tech tree after the player has had a few run-ins with the behemoth enemy hinted at in Factorio Friday #413. There would be a harpoon turret, and maybe a handheld speargun as well.Brain harpoons would be shot into the braincases of organic enemies with a meaty thunk and a brief spray of alien blood, there to stick out, a red blinkenlight on the end to signal its activation. The freshly lobotomized unit will now be friendly to the player's faction, albeit significantly stupider. A radio receiver in the harpoon will allow harpooned units to be controlled by the RTS tool.
Why?
Thematically, the brain harpoon would be a mirror to the recently unveiled Tesla Turret, as Gleba and the Behemoth is a mirror to Fulgora and the presumably robotic enemies there. Where Fulgora is all about mastering electricity and repurposing technological scraps, Gleba seems to be about mastering biology and nature around you. And in true Factorio fashion, this is being done by paving over the pre-existing ecosystem with monoculture farming and bioreactors. The brain harpoon lets the player extend this mastery to animal life as well, enslaving the locals to the will of the machine.From a visual standpoint, it would be possible to implement mind-control via pheromones or a modified pseudo-cordyceps produced out of the Biochambers, but this is insufficiently tech-y and bailey-wired for the Engineer's style, and it would be difficult to signal that the enemy has been mind-controlled. A brain-harpoon, or crude cybernetic controls, would be impossible to miss or misinterpret.