Don Carlos has inadvertently challenged you to defeat his younger self in Old Hillsbrad. Afterwards, bring Don Carlos' Hat to him in Tanaris as proof.
My hat? Yes, it's an unusual style, I know, from far away... few tailors here know how to make them.
When I was young and hot-blooded, I wore it as a standing challenge. Many came seeking to defeat me and win it for themselves... but no one ever did.
Ah, but those days are long past. I'm an old man, now: I drink, I fish, but I no longer duel.