There WAS an organisation called the Arcani in the Roman Empire. As far as I know, they were a secret police unit rather than a battlefield unit.

I don't think peasants are ahistorical at all. In fact, we use them in a VERY historical way - to garrison cities.

Whenever someone attacked a city, of course sometimes the town militia would try to engage the enemy away from the city. However, if it came down to a siege, with soldiers going over the top, or a last ditch sally, there would absolutely, definitely, beyond a shadow of doubt, have been peasants around. All the old men and boys would be given whatever vaguely resembled a weapon. After all, if they fought, they might win. If they didn't, and the enemy won, they wouldn't live either.

Maybe they weren't peasants in the sense that they weren't farmers. But they were certainly ordinary guys with either no training or some physical disability or old age upon them.