1 Adam: So, chess is out, huh... How about a different challenge for my life?
2 Adam: I challenge you to a contest of... Mythbusting!
3 {Head Death considers this}
4 Head Death: YOU'RE ON.

I know there are many non-native-English-speakers reading this comic. I hope the Head Death's last line is understandable to all, but just in case:

"You're on," is an idomatic way of saying "I accept your challenge/bet/whatever," with additional emphasis on being pleased to do so.

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2016-03-08 Rerun commentary: With the rise of computer programs able to play games such as chess, and now go, better than humans, we will soon see a cadre of robotic intelligences that cannot be killed because they can simply defeat Death when any attempt is made to shuffle them off this mortal coil.

This is the real danger of the humans-versus-machines dystopian future scenario. Once we program the machines to play better chess and go and other games than any human - we'll never be able to kill them.