Actually, that would be Demon’s Souls. Dark Souls is the spiritual successor to Demon’s Souls for the Ps3. Demon’s Souls is the spiritual successor to the original Castlevania for the NES.

Actually given that the original Castlevania is entirely linear it doesn’t fall into the “metroidvania” structure that Demon’s/Dark Souls share so it might be more appropriate to say that they’re a spiritual successor to Symphony of the Night. However, they do have the Castlevania 1 feel of attacking slowly and having to take your time, so perhaps the right thing to say is that it draws inspiration from the Castlevania franchise as a whole, particularly the ones deemed most classic.