The path to Mother's Crown is far. You will need to override a Strider. Find one and sneak up to it so you can successfully override it by pressing and holding TRIANGLE. Once you have a Strider you can mount it and ride it all the way to Mother's Crown.
Talk to Varl
Along the way you will encounter Varl near the gates of the Embrace. You must speak to him to start a cutscene.
Talk to Marea at Mother's Crown
When you enter the Mother's Crown, Marea will be in the center of the village. Talk to her to initiate a cut-scene. You will unlock a new quest the War Chief’s Trail.Ignore that quest for now.
Clear two Corrupted Zones
After talking to Marea you will have to clear the two corrupted zones that she mentions. Follow the quest marker to locate the corrupted zones. Prepare with fire arrows and traps to make quick work of the Corrupted Machines
Go to the Carja Border Fort
When you get to the Fort, a Corruptor and some Corrupted Machine will be there. Kill them and you will be able to enter the Carja Border Fort. Talk to the commander of the Carja Border Fort and complete the quest.