It hasn't fully unfolded yet, and I wouldn't know what it is, except that I discovered one (from the same bulb, probably) in full bloom in my yard a few years ago and people on the internet told me what it was. "Surprise Lily" or "Storm Lily."
The last time was also September, in 2005:
I'll bring a photo of the new one when it's opened up.
I bought a dozen bulbs of that and put them in the back, but none has come up yet. Judging by this one, having come up twice in ten years, I'm not expecting them every year.
It took until Friday afternoon for it to open, and I wish I hadn't missed the half-way point. Holly took these photos and that's her hand, for scale. This one was shorter than the one in 2005, and it didn't open as quickly.