Vividos has sent you on a pilgrimage to the birthplace of Moriens to "pay homage to Moriens and hope for a visitation". The only given clue to its location is that you should "seek out the signs that mark the way" in the Upper Catacombs.
Go back to the Upper Catacombs the same way you did before - by casting Open Ground at the building north of the monastery. Once inside, follow a similar course as before by first taking the north passage, then going east beyond fence and skeletons and then all the way east to the pile of skeletons and northwest to the fence with the "Danger do not enter" sign. Go on the north side and walk through the metal wall by the red pillar, which you lowered last time. Go east, and the building you want is in the second turning south. If you have the patch, the sign beside the door will read "The Birthplace of Moriens"; if not it will read "Towards Fate do you travel".
Unlock the door with the Key of the Scion and enter. The north turning is a dead-end with a useless altar and some lights, so go west. Jump over the lava and go west to a dead-end with some rising ghouls and a "Rest in Peace" sign. Use the Restore to Sight scroll on the skeleton to make a chest appear under the plaque. It is full of explosives, and has magic armour and seven different potions.
Go back to the lava and go northwest this time. In the backpack is a book on Zealan mythology and a scroll hinting that you are just about to enter a Zealan Shrine. Go north of the backpack through the doors and into the shrine.
Go north past rolling spiked spheres and a door, and east past a north turning to five levers arranged as if on the points of a pentagram around a fenced chest at the centre of the imaginary pentagram. Just north of there is a pair of massive locked doors. The idea is to pull 2 to 4 levers in the correct sequence to lower the fence surrounding the chest, which contains the key to the locked door. The consequences of pulling the wrong levers is either inverting the screen (without an immediate remedy) or unleashing Avatar-seeking fireballs. The solution is random, but I believe that there may be a fixed sequence of things to do, hinted at by the computer. This is partly because of the pentagram arrangement of the levers, but also because the Avatar will usually look in the direction of the next lever you have to pull (if he does not, you usually need to pull the same one you just pulled). Unfortunately, this does not always work, but I got it to work most of the time. You will have to solve the puzzle by trial and error using these hints, until someone finds a fixed solution. (Update 26th May 2023: see the Ultima 8 walkthrough by gibberishh for the puzzle solution.)
Once you solve the puzzle and the fence is lowered, unlock the door with the key from the chest. Once you go in, the only available path will take you to a dead-end with a trapped chest. Grab the Skull of Quakes from the chest and keep it for later used. You can also pick up cash and reagents from the floor and the skeletons.
Go back to where you skipped the northern junction, and this time take it. You have now come to the fence maze. Gates in the fences will sometimes lower when you approach them, but will sometimes also seal your way back. Pick the gates east, north, west and west again so that you come to an elevated oassage. Climb onto it, take two or three steps west and jump north out of the maze.
Follow the west path and climb onto the elevated ground. Jump over the water pit and you will end up at the tomb of Amrezhar (whoever he is). On a table you are given a bag and reagents to cast Open Ground; do just that, and the ground before the tombstone will fall apart. Jump into the hole and drop down to the level below.
To the east is a Zealan Ceremonial Shield, which is blocked off by stalagmites. You can only reach it later via the doors beyond the stalagmites. Go west, north, east and east again until you see a table and floor filled with rolling spheres (like marbles). You need to throw an object on the raised pressure plate just north of the fence in order to lower the gate. From here you can collect 2 keys, reagents, lots of cash (under a skull candle in a chest) and other goodies.
Jump over the beam to avoid injury, and go east. You have just come to a classic puzzle in programming - the Towers of Hanoi, just that you will need your mind to solve it, not a program. This game involves three staircases - a complete one of four steps on the right and two places on the left and in the middle for temporarily putting steps. The objective is to build a complete staircase in the middle. When you step on a pressure plate in front of a staircase, the first step will be lowered, and will be brought to the staircase where you next step on a pressure plate. You cannot, however, have a bigger step in front of a smaller step. This is not easy to solve without help in a game like this, but try it out physically by using four different-sized coins and it will all be much simpler. This is the solution just in case you can't or don't feel like solving it yourself:
The red and blue beams now blocking your way are death beams. You can only go through them alive by ingesting a purple potion, or casting Rock Flesh or Withstand Death. After the beams, you finally arrive at the Zealan Temple.
Unlock both doors and take the key under a skeleton on a bench. This is a master key that unlocks many small double-doors throughout the shrine, allowing quicker access to various places. The statues in the temple will not yet speak to you because, as is described in Yradan's book, you need to show them the Zealan Ceremonial Shield you saw earlier.
Leave the temple and go through the east doors, unlocking them with the key you just picked up. Do the same with the doors to the south and you're back at the Towers of Hanoi puzzle. Go west beyond the south turning with the quake beam and there are more doors to unlock in the south direction. Unlock just one more set of double-doors south of here, and you've gotten to the Zealan Ceremonial Shield.
Find your way back to the temple (north all the way, east all the way, north, west, north, west). Place the shield on the altar, and the Zealan deity statues suddenly become animated. They tell you to get a pyramid-shaped object made of blackrock from the Tomb of Khumash-Gor, which is the room behind them.
Use Mythran's scroll to unlock the doors, and go in. Khumash-Gor's ghost will attack you; kill it with magic weapons or Grant Peace. It will drop the Scimitar of Khumash-Gor. From the small boxes in the back you can pick up the Obelisk Tip (the required item) and a keyring (most likely the first one you'll encounter if you don't have the patch). The chest contains some healing potions. It's curious that you'll also find some Tenebraen Ale... Khumash-Gor after all was a Zealan, and is not likely to have gotten Pagan alcohol as a birthday present.
Go back to the Zealan deities. They will speak important information about your quest. They will talk about the Fifth Element (Ether), and how you must become its Titan by stealing the powers of the other Titans. Finally, you are set to seek the other Titans - Hydros in her underground temple and Stratos and Pyros who dwell on the surface. No clue is given about where to find their dwelling places.