For those who don't know, the upcoming DLC will continue where the game ending leaves off. The following is from a twitter discussion between vgarabia and producer David Cox.
@CastlevaniaLOS I finished Castlevania:LOS today. I really liked the ending, it make you curious to see what next for the franchise. I'm very curious so I want to ask you about the upcoming DLC: Will the upcoming DLC for the game continue from where the game ended?
Yes, this sucks... and they are doing the same **** that ubisoft did with prince of persia... DLC is going to low this days and they just do not even release complete games anymore. Now you need to buy the game real ending... I did not bought the one from prince of persia and I don't think I will buy this one.