Erend's sister Erse was ambushed and killed by the Shadow Carja. He wants to track down the soldiers who did it and take his revenge.
If you didn't do this quest immediately that is fine, but you will have to complete the quest to progress in the story line. Make your way to Red Ridge Pass and meet Erend. When you get there, you will have to fight some machines and then talk to Erend to discuss his sister's death.
Follow Erend to the battlefield. Scan the area for tracks and then follow them.
When you get to the end of the tracks you will be ambushed by attackers and machines. Take them out and scan the area for any clues to Erend's sister.
Once you've scanned all the clues, Aloy will come up with a theory. Talk to Erend about this and a cut-scene will start for the next quest: Into the Borderlands
