[vfb]

I think it's more of a silly trick. Ineffective and silly. And it's expensive too (you lose the item of course), and you've got no idea who's going to try to hire the merc. But it does have some entertainment value.

The Bane Venom Charm, at least, will affect your own province and units at the lab where you equip it, in the last turn the mercenary is yours. So, better not do it in your capitol if you don't have healing/recuperation/all undead units.

The Bane Venom Charm will not affect the province it arrives in on the turn the Merc is hired. So even if Dagan arrives in the capitol, the enemy should be able to move him out into a province where he'll have less disastrous consequences. He'll only poison the province he moves to, not the capitol. Even if you siege the capitol with undead troops on the turn you lose the Merc to the enemy, the Merc can be set to break siege and still not poison everyone in the castle (assuming he dies).

Other than that, the only cheap way to sabotage a Merc is to give a slave collar to a Merc mage. Or I suppose you could give a Berserker Pelt to Sanne, but the enemy could still get some use out of him.