Menno Coehoorn is one of the best cards Nilfgaard has. He is a strength 10 Close Combat hero, so he is immune to weather and abilities, and he also has Medic: when he hits the table you revive a non-hero unit from your discard pile and play it. The flip side: a Commander’s Horn won’t boost a hero, and a Medic can’t bring one back, since Medic only revives non-hero units.
You win him from the innkeep at the Inn at the Crossroads in Velen, one of the opponents in Gwent: Playing Innkeeps.
