This section of the guide to GreedFall has the walkthrough for In Pursuit of the Tierna Harh Cadachtas, one of the main missions.
Go to the altar in front of the gate and examine it. Then, go back to Ongos and ask him about how to get to the passage made from roots. The safest thing to do is to use Charisma. Go inside and search the house tierna harh cadachtas - your attention is drawn to the flower, whose seed can help unlock the passage.
Find a similar flower near the village, collect the seed and run to the altar. The trick will work - the passage is open. Run forward - one of the paths leads to an altar with a scythe - this is one of the legendary items that you can take.
finally, you will come out of the cave and reach a new area -Frasoneigad. Go to the destination - you will fight with Nadaig Frasamen. You now have a choice - you can kill the spy or spare him. Run to the wounded woman and move her to the village.
This completes the quest. You get +2,000 XP. You will also start Trial of the Waters quest.