There's an icon that appears on the scorecard that must have something to do with treasures we collect throughout the event. Not sure how it works though.
Also, Terrorblade seems fun. His attack speed is great and his animation is one of the best I've seen. I can see why S&Y Skadi is recommended though, with my Q plus them it was like they were standing in Warlock's slow.
Edit: I also tried Ability draft and the way it works is each of the 10 heroes picks one skill at a time (mostly, at one point it looked like I could pick two but I wasn't sure). The time to decide is really really small, you really have to make your decision quickly. And if you don't then it randoms one for you. Only allowed one ulti, and you can pick the ulti in whatever round of picking you want so it may be the one to go for first.