Dervish Scythe Sin(How to get?)

I'm just wondering how does one get an assassin subclass, or vice versa if your an assassin wanting a dervish subclass? I have factions, nightfall, and EoTN. So I Should be able to get the classes, I'm just curious how to.

You can only initially pick a secondary available to the campaign in which you belong to. That means when first picking in Nightfall, the Ritualist and Assassin professions are not available. You can change to either of these once you have completed the Hunted! quest or otherwise ascend in Prophecies or Factions. Just talk to the NPC in the Great Temple of Balthazar and you can pay 500g to unlock each profession.

Ariena is right. At first you can't choose D or P secondaries, but once you Ascend (http://guildwars.wikia.com/wiki/Ascend) you can talk to any profession changer (http://guildwars.wikia.com/wiki/Profession_Changer) and he can change your secondary to any other of your choice, at the cost of 500g.

Then, you can freely change between any secondary professions you have at no cost, as long as you are in any outpost or town.