Aconitum aka Monkshood.

Here is a link that might be useful: Aconitum

Carol- that is exactly what it is. I didn't plant it, so it much have come from the wind? A bird? Probably not a bird since it is so deadly.
And I've been handling it like a benign entity. I even brought it in for the winter!!!!
I guess I'll dump it into a black plastic bag. As pretty as it is, I have no desire to husband a killer plant.

I can't thank you enough!

I grow Aconitum and think it's beautiful. There are many toxic plants in this world. Unless you have concerns about a child eating it, or you have a pet that browses on plants, it's not a problem.

which one? arendsii? It is a great perennial...

@flowers
As carol says unless you have kids don't fret. I would guess that if you did a full survey of your garden and surroundings, you will find that there are lots of potentially toxic plants surrounding you.
Even the common daffodil bulb, if mistaken for a onion, can be lethal.
Just enjoy but be aware that beauty can sometimes come with a sting in the tail.

If you decide you want to give it a half a chance at survival, put it back outside, preferably planted in the ground, ASAP.

Since my plot is so tiny, and there is a small likelihood that a smaller child could reach over and grab some of it with no problem, I am unwilling to take that chance.
Directly in back of the house is a children's park that gets heavy use. If some of the seeds flew that way and germinated...
So this morning it was dumped into a black plastic bag, and there it will stay, until the dirt dries out and then will be disposed of.

I love foxglove, but have never grown it. Maybe it's because I'm an overconscientious nurse that I've avoided planting known poisons.

Thank you all for your comments.

I still can't figure out why the purple poppies failed to germinate at all, since that was what was supposed to be in the pot!