The illustration of Hero’s Poliorcetica shows the Cheirosiphon throwing the ignited substance.
The immediate question is, how is this going to be implemented in game?
Cheirosiphon (Petard upgrade). It Will be available after researching the already existing Castle age Unique tech Greek fire, additionally to its current effects.
Its exact design is up to conversation. Petards are slow moving (0.8 - also Teutonic Knight movement speed), and if it will not be for one-time use, then it can have a range of 2-3, competing with the slower siege units (0.6) of the opponent and their buildings (with attack bonuses), damaging over-time instead of immediately, like a Fire ship on land. With a proper design, can be a magnificent defensive tool in the hands of the Defensive Civilization, iconic for the game, utilizing more convincingly on land the Castle age UT.
I don’t forget the other popular proposal, the heavy infantry unit Varangian, but the idea of Cheirosiphon recently comes up hot like its spout substance.
The fact that it upgrades Petard but it is not of one-time use, is problematic tho.
In the comments below another idea has been proposed; a short range, fast firing, Mangonel-like siege unit (with the appearance of a footman carrying a flamethrower), with attack bonuses against siege units & buildings (wooden things, so “stone defense” armor class should be excluded), unlocked in Castle age, after Greek fire UT research, trained at Castle.