Children of the Corn

I have two very strange plants growing side by side in my backyard. For a long time, I believed it was corn, from a birdfeeder or a messy squirrel. Until a) the plants didn't produce any ears of corn, and b) where the feathery bits at the top of the cornstalk usually are, each plant has produced a clump of hundreds of tiny, gray-green berries.
I have scoured my field guides and searched the internet. Apparently, this plant exists nowhere but my backyard. Which disturbs me. Is it poisonous? Will the tiny gray-green berries burst open to reveal hundreds of tiny alien babies?
Totally freaky.