Valve has now confirmed the details for its Dota 2 Nemesis Assassin event, which introduces new hero Oracle and spotlights Phantom Assassin.
The event introduces an extra element to some of the game modes, if a player owns a particular item set (The Manifold Paradox Arcana), where Phantom Assassin (PA) is tasked with killing a particular enemy hero from a list of potential targets. If she succeeds, her whole team is eligible for a trophy and if the target kills her first, the opposing team is eligible for the bonus.
The trophy will only be handed out if that team goes on to win the match. I'd assume that's a decision Valve made to mitigate concerns that players would ignore the overall match-winning objectives and gameplay in order to get contract bonuses.
If they fulfil or deny the contract and go on to win the game, then there's a chance to trigger a drop of items for the winning team – it looks like that chance goes up the more Arcana owners are in a team and the more games those arcana players have played as part of the event.
From reading Valve's post, it also looks like the Arcana owner doesn't even have to be the one playing PA in order to trigger the event in a match. The only thing that matters is that there's a PA on one side and a hero from Oracle's target list on the other team.
If you're wondering about the lore explanation, it's approximately thus: Oracle can see into different timelines and has made a note of heroes who kill him in those instances. He's compiled them into a list and passed said list over to Phantom Assassin.
Before anyone freaks out, the event is only available in unranked modes excluding bot matches.
As for who's currently on the list?
The Nemesis Assassin event runs until 7 December, 2014.