I gotta step in and defend this guy because no one would be 100% certain it would 'stack through out the run and trigger at start of game' just by reading it. It would be an educated guess. The 'when you fight it' part sounds like you have to do something during the fight. At least cast it and then get the bonus 10 damages on that cast. Something like, 'deal *40* damage' when youre facing the boss.
I am going to defend the OP on this one, the card description certainly does not make it feel like its effect stacks. And seeing how OP submitted a Non-English screenshot, it's safe to say Eng is not his primary language, we also have no idea if the effect is properly translated into whatever language OP is playing in.
When I first picked this treasure I thought it would automatically deal 10 damage to the Plague Lord at the start of match. And it turns out it didn't because I haven't casted it a single time in the run either due to bad luck or the damn Sinkhole ate my cards.
To be clear, every time you cast this spell during a run, it counts as an individual proc of the card's text.
So, if you cast the spell 3 times in total for a run, then there are 3 separate iterations of that spell that are in effect. And it clearly states that they deal damage to the Plague Lord when you fight it, which pretty much answers the "autocast" query. It feels like this is a bit of a non-issue in fairness.
When you cast a spell, whatever's on the card text happens. Easy enough to understand.
If you cast a spell multiple times (say, against multiple bosses in one run), the effect described in the card text will happen multiple times.
I could understand if you weren't sure what would happen when the treasure was used. But when the effect actually happened you weren't expecting it and for some reason you didn't just accept that's what the card does.
Too much sad kids that can't go trough rank 10 and come here to be salty or sick somehow. I'm a sicko too but at least I reached legend sometimes...some here never really got far than 5, and some are really bold to stream their shit. xD
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.