Shadow of the Tomb Raider

First of all, the final DLC mission, The Path Home, is only avaliable with the Season Pass.

The 7 missions / challenge tombs you have to complete are:
1. Echoes of the Past, started in Kuwaq Yaku. This completes the challenge tomb, Forge of Destiny.
2. The Age Makers, started in Skull Cave in the Hidden City. This completes the challenge tomb, the Path of Huracan.
3. The Path of Fear, started in Skull Cave in the Hidden City by talking to Jonah and Uchu. This completes the challenge tomb, the Howl of Monkey Gods.
4. The Dragon and the Prince, started in Skull Cave. This completes the challenge tomb, the Sixth Seal.
5. The Two Graves, started near the Village Caves base camp after rescuing Hakan. This completes the challenge tomb, the Slayer's Gauntlet.
6. The Wrathful Gods, started near the Mission Gate base camp in San Juan by talking to Jonah. This completes the challenge tomb, Zipacna's Craving.
7. The Path Home, started in San Juan by talking to Abby. This completes the challenge tomb, the Mother Protector.

Each DLC mission and challenge tomb has an achievement tied to it. Upon completing a mission, you will be rewarded an outfit and weapon, that upon equipping at a base camp, will unlock another achievement.

Upon completion of all 7 DLC missions, you shall recieve the Tunic of the Shorn One. I did not have to equip it for the achievement to pop.