Phoenix is the latest hero unveiled as part of Valve's upcoming Dota 2 New Bloom event, which will also see the introduction of a new feature called Takeover Mode.
Takeover Mode lets players to take control of any replay created post New Bloom update, allowing them to replay the match from any moment. Other players can join as any existing hero in the match via a lobby. A Takeover will spawn new replays as they considered a new match and are recorded once a player takes control.
Earlier this week, Valve announced new hero called Terrorblade, a "demon marauder" whose abilities include Reflection, Conjure Image, Metamorphosis and Sunder. Random Ability Draft was also announced where the game randomly assigns a hero to a player and lets them choose abilities from a pool.
The New Bloom page contains further information about hero additions and new features that will be introduced with Game Update 6.80, the roll out of which will coincide with New Bloom event beginning Jan. 31.