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.