Thalia, Heretic Cathar
Spireside Infiltrator

At FNM last night, someone had Thalia out and their opponent cast Spireside Infiltrator. Neither one knew whether the Spireside Infiltrator would trigger, so I got called over to judge. I wasn't entirely sure either. My gut feeling was that it wouldn't trigger, but because it was a weak gut feeling I went ahead and ruled that the Spireside would trigger.

As the night went on, it really started eating at me that I had made the wrong ruling, but as nobody else was running white at the event and only the one player had Spireside Infiltrators in his deck, the situation never came up again. And the Spireside player lost the match anyways, so they clearly weren't given an advantage for any wrong ruling.

However, I'd like to know for sure what the ruling on this should be in case it comes up again (which it might, local players tend to have favorite deck archetypes that they run). And I did let both players know that I would make sure I looked up the interaction in case I was wrong.

It wouldn't trigger:

Ok. Not sure how I overlooked that. Thanks.