There are currently two different types of NPCs offering rewards:
The Priests of Balthazar in the Core areas allows you to exchange Balthazar Faction to unlock skills, runes and weapon upgrades. These Priests of Balthazar (some are specifically given a name) all have the suffix "[Rewards]" tagged to their name.
See also: Category:Faction Rewards
Introduced in the Factions Campaign, the Kurzick Bureaucrat and the Luxon Scavenger allow you to spend your Kurzick Faction or Luxon Faction points. These two NPCs have the suffix "[Faction Rewards]" tagged to their name, and you can find them in your Guild Hall, in every Alliance Battle outpost, in Fort Aspenwood and Jade Quarry, and in House zu Heltzer or Cavalon, respectively.
As noted in the individual NPC pages, you may spend faction for rare crafting materials, Passage Scrolls, Kurzick/Luxon skills or Alliance Faction regardless of your alliance's faction, although you will lose an equivalent amount (until zero) of the other.
See also: Category:Faction Rewards (Factions)