It's pretty hard to find it, so adding Echo support for notifications would make this feature more discoverable, along with making events related to it easier to track:

• $1 decided that $2 is the winner of this challenge!
• $1 decided that you're the winner of this challenge!
• $1 requested to challenge you!
• $1 accepted your challenge request.
• $1 denied your challenge request for the following reason: $2.

Maybe not the exact wording, but perhaps similar? There's probably some other events I'm missing, so feel free to add it if you think of one. :)