Kingsmead
The recipe for Kingsmead has existed in European occult circles for at least two thousand years, with significant gaps. The brew can only be made from honey from a beehive on Crown lands (however you define ‘Crown’): when properly prepared, it gives the drinker the ability to shrug off pain and wounds (half all damage taken, and damage regenerates in one-third the usual time) taken while in the Crown’s service. Traditionally, it was given to only the most loyal royal bodyguards and household troops, and those who took it kept it secret.
Too secret, apparently: many of the realms that knew of the existence of Kingsmead lost their knowledge during the Enlightenment, and the chaos of the Napoleonic Wars did it for the rest. The one surviving recipe languished in an obscure Polish library until the Germans looted it during the Second World War, and was only accidentally discovered by British scholars cataloging stolen Nazi loot in 1946. It took NATO occultists some time to reverse engineer the nation-specific rituals and spells needed to make Kingsmead work, and unfortunately there’s no way to industrialize the process.
On the other hand, apparently this process works just fine for non-monarchical nation-states, too. Interesting bit of trivia, by the way: it’s proving to be quite the fad to have official national beehives, very much including the USA. It’s also proving to be quite the fad to start putting serious albeit unobtrusive security on the hives, given that items with a symbolic occult link to heads of states can be both very valuable and very dangerous, in the decidedly wrong hands. Bright side to that is, getting the hives back certainly counts as “in the Crown’s service.”