The Cultivator is one of the Precursor devices found among the ruins of Karath-din. Originally used for agriculture, the cultivator was duplicated by Karras with the intention of using it as a means of spreading his Rust Gas over vast areas. Karras had one installed in each of the "party favour" Masked Servants given to guests at Angelwatch, all of whom had large estates with equally extensive gardens (it is unclear why the Rothchilds were invited after all, seeing as they live in what appears to be a modest flat far from any garden). Garrett had to break into the Gervaisius Estate to steal the original cultivator on display there.
The Rust Gas would be released through the mouth and reacted violently with plant matter. Viktoria assaulted Soulforge Cathedral, sacrificing herself to spread her Green throughout the Cathedral. When the Servants were recalled to the Cathedral, the amount of greenery caused a chain reaction big enough to reach, and kill, Karras.