What is it? I've picked it up frequently, occasionally multiple times, but can find no mention of it in the original Hexen manual nor in the Korax docs. Activating it produces no noticeable results.

In original Hexen it appears only in Co-operative mode. It's sorta like the flechette in that it has a different effect for each class. For the fighter, it adds 1 to your armour. For the cleric, it adds around 50-80% health (it's random). For the mage, it's a mana boost.

Thanks. I'd assumed it was part of Korax since I'd never seen it before. I should also assume its effects are temporary?

It's been years since I last touched KMOD so I'm not sure, but I think it had some kind of special effect in KMOD 2.x single player.