Whenever a new set is released, it's important nowadays that the cards and mechanics in it get integrated smoothly into Magic Online. As with all software, therefore, the Magic Online cards go through a beta test to iron out the bugs. Tonight, it seems, MTGSalvation Rumor Mill user whatisfgh has obtained closed-beta screenshots of Magic Online versions of every card in Theros.
Yes, this means the set has unofficially been fully spoiled about a week before the full spoiler list would ordinarily go up. This is nearly on par with the early spoils of the entirety of Return to Ravnica or the infamous Ruel-godbook leak of New Phyrexia. This is a massive spoil and processing all the new information will be a, well, Herculean task (reference unintentional but apropos), so expect analysis of the full set sometime in the near future.
In the meantime, either check out everything set one of Theros Block has to offer - or stay far away from every Magic discussion board if you want to be surprised at the prerelease.