The Flakbûn was a primitive mechanical (and presumably magical) weapon similar to a modern flamethrower.It was developed by the mordorian Machine-Tribe and usually only wielded by Olog-Hai Captains.Luckily they were heavy, hard to build and extremely squandering of material and resources, so they never became very widespread.
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This weapon appeared unnamed in Middle-Earth: Shadow of Mordor. Fantastic fire-weapons used by the Númenoreans were alluded to in the History of Middle-Earth but never described in detail.