Konami's confirmed that a Symphony Of The Night stage is to be added to Castlevania: Harmony Of Despair as PDLC this month.
The news comes by way of Konami's Japanese site, and it's uncertain whether or not the DLC will make its way west. We'll ask Konami.
The addition to XBLA's Harmony Of Despair will take the game to eight levels, the new stage, as Joystiq notes, being based off the underground area from Symphony of the Night - a level which culminates in a fight with spherical corpse-monster, Legion.
Both this level, and another titled "Lord of the Flies," will cost 320 MSP.
There's no specific date.