My team:
Giant Spider***
Hero, Crescendo (magic link)
Valkyrie***
Keeper of Souls*
Opponent team:
Valkyrie***
Valkyrie***
Valkyrie***
Valkyrie***
I made a skull match and all 4 of the opponent’s troops became poisoned. I’m guessing that when Spider’s Envenomed trait fired instead of just hitting the first troop, it hit all copies of that troop.
It’s more complicated than it seemed. This time I was fighting goblins. First goblin had been poisoned, was down to 3 life, and I used Crescendo (10 damage to each enemy). It killed the first goblin (which grants me an extra turn), but as it hit the rest of the goblins, the Spider triggered and poisoned the rest.
By spider do you mean Webspinner?
Nope, fully traited Giant Spider. Which is why it struck me as wrong since my team has no legitimate way to poison the 2nd-4th troops. The sequence went like this:
Match skulls, bringing enemy 1st troop down to about 8 life and poisoned via Giant Spider envenomed trait.
Fill and cast Crescendo on hero, which does 10 true damage to each enemy and if it kills any I gain a turn.
Crescendo kills the first troop, I get the extra turn message. Crescendo then delivers 10 true damage to the next troops. However, the Giant Spider’s envenomed trait triggers for some reason poisoning them. Repeat on 3rd and 4th troops.
If I had to guess, I’d say that the extra turn triggering interrupted script and the rest of the damage defaulted back to “skull” damage instead of “spell” damage, which allowed it to trigger envenomed. If so it would likely work with other cases: Maw’s devour, Archer’s kill, Sapphira’s frozen, Sheggra’s burn, etc…
To complicate things, it doesn’t always happen, so it is hard to test 
I’ve been trying to reliably reproduce it and I can’t.
I had something similar with fully traited Cthyryzyx - he fired his spell, shuffled the board, skull attack animation triggered (just animation, no skull damage) as he hit my Valk. and dealt damage at the same time applying Death Mark.
I even have screenshot of that - you can see that there’s no legitimate way for Death Mark to be applied on my last slot (well, any slot except the first one).
As @Rasper wrote it doesn’t always happen - it was the second time Cthyryzyx fired his spell in that battle and the first time everything went fine. AFAIR he didn’t shuffle the board though - I don’t know if that’s important or not.
