it depends on how you finished the war, this is after coming back from the mountains (taken from my guide):

• Neutral route: it unlocks automatically when you come back.
• Bull route: it can’t be unlocked, but Axel rewards you with an imperfect rune necklace, not as powerful as its fixed version (see below), but still gives +2 INT and +3 MATK, great for magic builds.
• Lizard route and Selye route: 
  • If you helped them set up the camp (back in the second part of the war): the moment you go up the stairs in the lizard tribe, advisor Red will talk to you about it and his and advisor Blue’s plans with it. There are 2 choices you can make:
    • Trading site: unlocks the campsite, after 3 days.
    • Defences: doesn’t unlock the campsite, but Blue will come out of the blue (yes, i wrote that~) after 3 days, in daytime, outside of the tavern or in the lizard village, to reward you for supporting him. If Selye was alive (Selye route), you’ll get a rune necklace, a very powerful equipment that gives +3 INT and 10% HP regen during battle (Selye even has some extra dialogue if you visit him while wearing it!); if he is dead (lizard route), it’s gonna be an imperfect rune necklace (see above).
  • If they didn’t set up the camp: it can’t be unlocked.