He was watching me fight some dragon in the Canadian province of Saskatchewan, where the mayor was recklessly letting the dragon do whatever it wanted thinking it had control over it with the help of some magical object, a tiara I think.
I had the ability to summon knives through my hands like Scorpion from Mortal Kombat, and I somehow summoned an entire army of undead before the fight started.
I got to phase "2" and the dragon started healing, and at this point Arthas showed up through the back entrance of the local drug mart which for some reason exited out from the mall through a mountain of ice, kind of like Wrathgate cinematic.
So Arthas takes out FM, looks at it as he brandishes it around, then hands it over to me. I go to the dragon who is now sitting docile in its nest and I just poke it lightly with FM and it doesn't do anything, like it's scared now.
I try to run back to Arthas to thank him but he walks back through the back entrance. I walk there too and I see the windows are broken from the mall side of the drug mart, as if people have tried to see in the past if Arthas was hiding in there.
So Jaina and I think Uther show up and I start walking back down towards the arena where I fought the dragon, and Jaina says "he gave it to you, he gave your Frostmourne", and I just raise it over my back as if to say yep as I walk away, then we sit on the trunk of some huge cut tree and I think I say to them something like "we need to discuss this important change of events" or whatever.
Then for some reason I start walking and I'm in the old Republic MMO and I think I don't have FM anymore but I'm thinking to myself gosh I hope this MMO has lots of side mysteries and lore to explore and then I think I wake up.