When adding a new discipline to an existing class (stalker, warder, and warlord as well as with pathwalker psychic warriors and war soul soulknives), each class will give a choice of what new discipline to add to its list of available disciplines. In the case of archetypes (such as the soul hunter archetype for the stalker class),
use the additions listed specifically for the archetype, not the base version of the class (in this above example, a soul hunter stalker would add Cursed Razor, and would not have the option to choose between Riven Hourglass or Tempest Gale).