Jägerbräu, or Jägerdraught, is a secret recipe of the Heterodynes, dating back to the time of Vlad Heterodyne the Blasphemous, responsible for the creation of Jägermonsters. This potion, distilled from (among other things) the waters of The Dyne, was given to only the most loyal and strong soldiers, altering their appearance (but not germline genetics) in alarming ways, and making them nearly immortal and almost indestructible, creating the perfect shock troops and companions of the Heterodynes of old. When it didn't just horribly kill them, of course.
The English Spark Doctor Haggard devotes much study to the draught, and is able to come up with a knock-off version that replicates the effects for a short period of time when consumed, turning him into the more powerful, intelligent and capable Lord Bunstable. Unfortunately, it soon wears off, leaving him doddering and somewhat senile, and the length of time the stuff works continues to drop with every dose. When Bunstable meets Agatha, he offers to formally take the Jägertroth, but she has not yet been instructed in how the process works, and so is unable to help him.
Der Jägerbräu (vit or vitout umlautz) iz a zecret rezipe uff de Heterodynez, dating beck to der dayz uff Vlad Heterodyne der Blasphemous, who vaz a veird guy amung veird guyz, but also a very schmot guy. Dis potion, distilled from de vaters uf De Dyne, vas given only to de most loyal uf his soldiers, changink dem into de first Jägermonsters. Dey hat become nearly immortal, und alzo nearly indestruktable, mekking dem de porfekt schock-troopz und companionz to de Bad Old Heterodynez. For generations, doze who svore de Jägertroth deed so in hopes uf vun day beink uffered de Jägerbräu,
Uff-duty, dhey iz fond uff beer und mekking jokez und flirting vit de pritty gorlz.
Vhen on-duty, dhey iz relentlezz hunterz und fighterz, schtriking fear into dheir enemiez und fighting un in schpite uff injuriez dot vould kill a normal man.
All uff diz, de vay dot normal men iz torned into monzterz (rumoured to eat babiez!) iz laid upon der doorschtep uff de myzteriouz Jägerbräu. De zekret uff der "bräu" iz klozely guarded, keeping de tektical adventage among de Heterodyne forzez.
Credit due to Jägerkapitän Veles JagerMunster; posted as an interdimensional courtesy.
Relevant Outside Information
There may be a vague connection between Jägerbräu und Jägermeister.