It appears Konami's leaked-pulled-and-then-leaked-again Castlevania: Harmony of Despair is very much a real thing. The rumored 2D multiplayer Castlevania game is part of the Microsoft's returning Summer of Arcade promotion. The junior year's games include:
The press release hasn't mentioned any added bonuses for purchasing all of the games, but previous years promised a refund of 800 Microsoft Points ($10) with purchase of all five participating titles. It's likely this year's Summer of Arcade will do the same, but we'll follow up.