Valve has just announced a brand new game during its Dota 2 International livestream. And fittingly enough, it's a Dota themed card game. Artifact will release some time in 2018, and that's precisely all we know about it at this stage. The trailer above, while pretty enough, doesn't provide any more clues.
I'll update this story when new facts come to hand. In the meantime, there's a Twitter account, which is already receiving numerous demands for beta access.