This treasure is crap anyways, don't pick it, unless the Plaque Lord is like at 20-30 starting health and you just wanna oneshot him
It's worth noting that this is affected by Spell Damage. Say for example you have Malygos in play, and the +2 spell damage Hero Power, then play this, it won't only deal 17 damage to that boss, but also to the Plague Lord. You can potentially skip the first phase of the boss by stalling in earlier fights, stacking spell damage on the board and playing this for high damage. Best I've managed so far is 82 at the start of the fight. It's still not an amazing treasure, but it's better than some. I would assume, but no confirmed that it works with Vargoth as well.