You'll find Tuls Valen just inside the door to the Ald'ruhn Temple. He has four quests, the first three of which have no rank requirements, although you must have completed the Pilgrimage of the Seven Graces.
You need to cure a Sheogorath worshipper, Bulfim gra-Shugarz, of a Blight Disease. She's in the Maelkashishi Daedric ruin. If Valen had been leading the Israelites instead of Moses, they'd still be wandering in the wilderness because his directions are hopelessly complicated and really do send you on a rather long and circuitous route. He wants to send you east from Ald Velothi and then follow the mountains south to the ruins. If you can levitate, there's a much easier way. Go to Maar Gan and then Levitate over the mountains to the west (it's almost directly across the mountains from the Hairat-Vassamsi Eggmine). You should come out right on top of it. Alternatively, take the Silt Strider to Gnisis (which you would have to do to get to Ald Velothi, anyway), use the bridge to cross the river to the east, then head east cross-country until you hit the mountains. It should be right in front of you. Or it's northeast from the Berandas stronghold.
Valen tells you that you should not go any farther into the shrine. gra-Shugarz is near the entrance and is not hostile, which cannot be said for the daedric critters in the ruins or the two worshippers near the back wall. You have a couple of options on how to deal with her. You can give her a potion (Valen gives you one, but would like you to bring it back) or you can use a spell such as "Rilm's Gift" (purchase it from Folvys Andalar, who is upstairs) or "Vivec's Touch" (which you got by activating the real Ash Mask when you completed the Pilgrimage of Justice at the Gnisis Temple) or "Aryon's Blight Cure" (which you got as part of one of his Telvanni quests). Alternatively, Hetman Abelmawia in Gnisis sells scrolls with a Cure Blight Disease effect for other people/creatures.
Once gra-Shugarz is cured, report back for your goodies. Sort of. Even in the patched version of the game, the dialog options point to a non-existent journal entry if you bring back the potion, so the quest won't terminate. But if you don't have the potion, you can't get your normal faction and disposition boosts. The Morrowind Patch Project (it's a plug-in, but it does require Tribunal and Bloodmoon) fixes this so that the quest terminates properly and you get your proper rewards. More tangibly, you get a skillbook: The Four Suitors of Benitah. Since you get the same book from another Temple quest, then either this quest or the other is a dud.
Elvil Vidron has declared himselve to be Nerevar reborn, which is purely heresy. Go convince him of the error of his ways. He's the shirtless fellow wandering in Suran near the stairs.
How this plays out depends on where you are with the Main Quest. If you have started the Main Quest and had at least one dream, you can convince him to cease and desist by getting his disposition up to 65. Note that if his disposition drops too far (it drops every time you fail to convince him), he'll go hostile and attack you.
If you have started the Main Quest and have retrieved the ring "Moon-and-Star", you will have the option to show it to him and he will stop and apologise.
If you fight him, you'll have to kill him (which will certainly put a stop to his heresy) and you'll get no reward for completing this quest. Otherwise, you get three Exclusive Restore Health potions. Note that this quest is also bugged in that it contains an incorrect journal pointer when you use the "Moon-and-Star" option (Morrowind Patch Project fixes it). If you are not using the plug-in, you'll either need to persuade (option 2 in his dialog) or fight him.
Valen wants you to complete a pilgrimage (doesn't everyone?). He doesn't tell you much other than to go to the Temple at Maar Gan, read the inscription and imitate Vivec. So do that. The Temple is the largest structure in Maar Gan, so it's hard to miss. The inscription, however, isn't so easy. Where everything else was written on the trilith, this inscription is on the big rock inside the Temple.
When you activate the rock, you'll find out that Vivec taunted Mehrunes Dagon into throwing the rock at Vivec rather than the people. And, coincidentally, there is a handy Dremora just waiting to be taunted. You cannot complete this quest with a preemptive strike. If you do, you'll have to wait for it to respawn and try again.
You must Taunt the Dremora into attacking you. But you do not have to actually fight it. Once the Dremora goes hostile, you can activate the rock again to complete the pilgrimage. This is not to say that you can't fight the Dremora, but only that you don't have to.
Aside from the blessing (Fortify Personality and Speechcraft for about a day), you get a skillbook, The Death Blow of Abernanit (Block) when you report your success.
You must be at least Rank 7 (Disciple) to get this quest. Valen would like you to wipe out a dark cult hiding in the depths of Hassour (follow the foyada south from Fort Moonmoth and it's at the end, on the right). You might have already done this. Hassour is the Sixth House base that's messing with people's minds in Balmora and Pelagiad.
The cave has two sections. You'll find a few leveled Ash creatures and a handful of NPC Dreamers, but you must kill Dagoth Fovon to complete the quest. Fovon is an Ash Ghoul at the back of the Shrine section, so you'll have to wade through almost everything else first. He has a 20% Spell Reflection ability, likes to cast Ash Feast, a Touch-ranged Drain Health effect (3 points for 30 seconds) and can infect you with Ash Woe Blight (Drain Intelligence and Willpower 20-40 points). He has a Sixth House Amulet if you're collecting/selling them.
There is a reasonable haul to be had
Your reward from Valen is a some Cure Blight Disease potions and scrolls.
This is Valen's last quest.