Tales of Arise

"Otherworldy Visitors" is a post game sub quest that is VERY long and challenging, consisting of 6 new areas each with a boss fight. The enemies range from lvl 60 to lvl 95 as it progresses, BUT don't worry too much since you will level up heaps during this quest.
It requires you to have finished the other sub quests from the researchers on Daeq
Faezol including "Another Hillside Anomaly."
The "Spirit Temple" sub quest is also required as it uses the same area.
Guide here: 
However I noticed the quest appeared for me after finishing the "Farewell, Mage" sub quest, so you may need to do that first as well. Guide for that here:

NOTE: The new enemies you fight throughout these areas will give you the materials needed to forge the strongest weapons available so make sure to create those when you can.

Start from "Space-Time Researcher" in Earth Spirit Temple who will give you the Destiny Key which will open the first rift to the Otherworld.

The Cavern of Fate is straightforward so just head around to the north (via the west) of the map after you've looted it. Find Edna at the end and obtain the Symphony Key after the fight which you can use for the next rift to enter the Spirit Forest.

The Spirit forest is a bit of a maze. There is good loot around but if you want the path straight through take the top middle path to the next map, then take the east path to continue on (the path west will take you back to the previous map). The 3rd map has 5 new paths to choose, take the bottom left path to reach the boss area. The boss "Nimus Origin" is not too difficult and will give you the Abyss Key.

The Abyss Rift will send you to a snowy area (Oracle Snowcap) that is easy to navigate through. At the end of the next map is the boss fight consisting of 3 tough zeugles that use different elemental magic. Rush them down one at a time until they are all down and gain the Zest Key for the next area.

Once you've arrived at Dragon Slumber Mountain you can loot the area for good items or grind some experience and materials from the lvl 75 enemies. When you're ready take on the boss Eizen. He is strong and can easily knock out your party members with his special curse move so try to keep your party out of the way and make sure to stock up on life bottles. After you're done take the Scarlet Night Key from him and head to the 2nd last dungeon of the quest.

Fight your way through the Volcano of Renewal and when you're ready step up to fight Edna and Eizen together. Yep, it's as scary as it sounds. Focus on one first and try to conserve your life bottles for a long fight. If you need more healing consider using Shionne and Dohalim together. When you take them down you will finally get the last key to open the Vesper Rift.

This rift will open straight to the boss arena where the final boss of the quest will appear, Chronos. I found this easier than the Edna+Eizen fight personally since there was only 1 target, but this boss hits hard too. Watch out for his large beams of energy and especially for his move that will slow down time so he can leave you defenceless and attack you. Watch for when he raises his hand and spam the dodge button so that you can stay invulnerable for the entire attack. Once he is defeated, congratulations you have finally beaten the quest! Head back to the researcher in the Spirit Temple to get the achievement.

P.S. you can return to the rifts after the quest is complete to grind exp for lvl 100. You can even battle Edna, Eizen, and Chronos again.