The Lost Valley domain is new domain in Chasm, that features 2 new artifact set:
Both artifact have +18% ATK Bonus for 2 set. As for 4 set bonus, Echoes of an Offering focus on increasing normal Attack. So character that focuses on using normal ATK like Kamisato Ayato and Yoimiya will benefit for this set.
Vermillion Hereafter focuses on increasing ATK after elemental burst while decreasing character HP. More HP decrease = more ATK buff, so character like Xiao will benefit from this set.
Where is Lost Valley Domain Located?
You can find entrance of Lost Valley domain in Fuao Vale which unfortunately sealed by some kind rock with Geo shaped mark on it.
Try to walk around the area and you will find some giant rock with yellowish element (similiar to Cor Lapis).
When you walk near the biggest Rock, Paimon monologue will appear telling you about the whole giant rock around the area is Geo Construct and when the big one releasing shock wave, the shock wave will resonate with other Geo Construct in the area.
Resonating Geo construct is the key to destroy sealed entrance. Unfortunately, shockwave from all Geo construct in the area can’t reach sealed entrance. What you have to do it, use one of your Geo characters to raise Geo Construct near sealed entrance. You can use Zhongli, Geo Traveler, etc.
I suggest to put Geo Construct on geo mechanism to spawn chest on top of farthest Geo construct. After that, head to farthest (and biggest) geo construct, hit it with elemental skill and wait until Geo construct resonates.
Resonation will happen, sealed entrance destroyed, and now you can farm new artifact in The Lost Valley domain. Don’t forget to go down to small cave near the domain to claim Precious Chest.