Elder Scrolls: Morrowind Guides and Walkthroughs
You can begin this quest line
Tienius Delitian Quest #1 - Speak to the People
Go to Llethan's Manor in Mournhold and raise people's disposition inside to 100 then ask them about King Llethan's Death. Eventually someone will tell you about The Common Tongue document and you'll get a Journal entry. There are 7 locations you can find The Common Tongue document but the easiest is on The Winged Guar's balcony in the Godsreach portion of Mournhold.
Tienius Delitian Quest #2 - A Temple Informant
This time we'll need to head over to the temple and speak with Galsa Andrano whose found in the Infirmary. Ask her about discontent in the temple and mention Mehra Milo and the Dissident Priests to get a Journal entry from her and the update we need. You may need to have done some of the main story quest to be flagged for this entry.
Tip: Almsivi Intervention takes you to the entrance of the temple
Tienius Delitian Quest #3 - Disloyalty Among the Guards
Ivulen Irano is found in the throne room near Tienius. Talk to Ivulen Irano about joining the Royal Guard and then mention King Helseth and King Llethan for a journal update, mention your Hlaalu connections while you're here too. Report back to Tienius and tell him what you learned then head over to the Guard's Quarters and speak with Aleri Aren. Mention joining the Royal Guard and then your Hlaalu connections.
Before leaving the Guard's Quarters grab the handwritten note on the ground by Ivulen Irano's bed, a screenshot of this note is below. With the note in hand return to Tienius and talk to him to complete the quest.
1. Talk to Ivulen Irano about joining the Royal Guard and exhaust all 3 dialogue options
2. Report back to Tienius and tell him what you learned
3. Go to the Guard's Quarters and speak with Aleri Aren on the bottom floor
4. Pick up the handwritten note by Ivulen Irano's bed
5. Return to Tienius
Tienius Delitian Quest #4 - Evidence of Conspiracy
Other Requirements: Chameleon or Sneak
Head on over to Llethan Manor, it's the northwestern most building in Godsreach. Inside you'll need to grab the handwritten note on the desk (pictured above), this time though you'll need to use Sneak or Chameleon to avoid being caught when you grab it. I recommend saving your game before you grab this note, sometimes it will agro the entire house even if no one sees you - unless you have good Sneak or Chameleon.
Return to Tienius with the letter and he'll give you writs to kill 3 people around Mournhold. Speak with each of them and you'll have the option of letting them go, Tienius gives you less gold if they get away but doesn't complain much otherwise.
Forven Berano: Almsivi Intervention places you right outside the temple and you can find Forven Berano a little to the northeast of it.
Hloggar the Bloody: In the northwestern portion of Godsreach you'll find a hatch which leads you down to the Residential Sewers. Go down and travel southeast until you find a ladder which leads you up to West Sewers. You'll find Hloggar in the rock tunnel portion of West Sewers.
Bedal Alen: You can find him upstairs at the Bookshop in the Great Bazaar.
Tip: The writs you're given for this quest can be used as get out of jail free cards for basically any crime you commit.
Forven Berano northeast of the Temple in Mournhold.
Hloggar the Bloody in the rock caverns of West Sewers.
Bedal Alen in Godsreach Bookseller's shop.
Tienius Delitian Quest #5 - Muckraking Journalist
Other Requirements: Sneak/Chameleon and able to open a lock level of 70
For the first part of this quest you're going to want to speak with Ten-Tongues Weerhat who is found in the Pawnshop located in the Great Bazaar. If you have a disposition of 100 he'll tell you about Trels Varis in the Craftsman Hall of Godsreach, the author of The Common Tongue. Trels Varis is behind a locked door in the Craftsman Hall so Sneak or Chameleon will be required.
When you enter the Craftsman Hall go immediately to the right and you'll find a door with a lock level of 70 titled "Keep Out!". For the most part the hallway behind you stays clear but occasionally someone wonders through. Save your game and open the door then head inside the trap door to find Trels Varis with a few more Dunmer in the same room.
Speaking to Trels Varis will give you a few different options to complete this quest, check the UESP entry for this quest for information about all options. Bribing him with 3,000G is the best course of action as it'll get you the King's Oath weapon from Tienius when you return to him.
UESP Muckraking Journalist Quest Entry