Oblivion:Blood of the Divines

Quick Walkthrough
# Talk to Martin and learn of the "blood of the gods".
# Explore the catacombs of Sancre Tor to find and defeat the first Undead Blade to learn about a curse.
# Defeat the three remaining undead Blades to free them of the curse.
# Kill the undead Warden to access the rest of the tomb, and watch the Blade Ghosts dispel the evil enchantment.
# Obtain the Armor of Tiber Septim and bring it back to Martin.

Detailed Walkthrough
The Blood of the Gods
Martin tells you the next ingredient to open the portal is the Blood of the Divines. Speak to Jauffre to learn that the blood of Tiber Septim (who became the god Talos) can probably be found on his armor in Sancre Tor. These catacombs were sealed by the first Grandmaster of the Blades and any who have tried to explore have never returned. Jauffre gives you a key to Sancre Tor and wishes you luck.

Sancre Tor is now marked on your map. It is located on the northern road from Bruma to Chorrol. Above ground, it is a huge ruined fort with a few leveled undead guarding a couple of chests and a Sidri-Ashak Rune Stone. The entrance to the underground portion can be found in the large central tower.

Sancre Tor
The interior of Sancre Tor is filled primarily with ethereal undead (ghosts and wraiths). Since these creatures are immune to normal weapon damage, you will need a silver, daedric, or enchanted weapon if you want to attack them. Alternatively, an Ancient Akaviri Katana will also kill ethereal undead, and they can be freely taken from the blade skeletons after you kill them. A Journeyman in Hand to Hand can use their fists to kill the ethereal undead.

You will also encounter several Undead Blades. The first one will turn into the Ghost of Rielus once you defeat it. Rielus tells you that he was one of four Blades sent to the catacombs by Tiber Septim to discover the source of the evil infestation. The four were captured by the Underking Zurin Arctus, and bound to haunt the catacombs after death. A spell was also put on the passage to the tomb where the armor lies. Approaching it is impossible without freeing all of the Blades. You will take constant Frost Damage, be hit with the "Breath of the Underking" which drains Speed and Willpower, and finally hit an invisible barrier at the end of the corridor.

Free the Undead Blades
Rielus tells you to find and free his three companions so the four of them can undo the curse placed on the Shrine by the Underking. Another Undead Blade can be found in the Catacombs, another in the Hall of Judgement, and the last in the Prison; killing them will release the ghosts of Alain, Casnar and Valdemar respectively. Note that each of the four Blades has a leveled magical item (Amulet of the Ansei, Valdemar's Shield, and two swords: Mishaxhi's Cleaver and Northwind). In the prison area you need to open a locked door: either unlock it yourself or kill the undead Warden Kastav for his key. When you have freed all four, proceed into the Tomb of the Reman Emperors to observe them restoring the shrine. Once the evil enchantment has been dispelled, go and get the Armor of Tiber Septim.

Return to Martin
Return the armor to Martin in Cloud Ruler Temple. He will thank you and let you know he's discovered the third ingredient. You may notice that Martin has set up another table in his impromptu study area while you were gone. There are two skill books (The Refugees, Light Armor and The Doors of Oblivion, Conjuration), a third non-skill book (The Firmament), an apprentice calcinator, and two paint brushes.

Notes
Followers will not follow you into Sancre Tor.
The Undead Blades can be looted for unenchanted armor and weapons as well. Though identical in appearance to normal Blades gear, they possess slight differences in quality. The Ancient Blades Shield and Helmet are worse (heavier, with less durability and protection) than the standard, but the Ancient Akaviri Katana is slightly better, as it is more durable, has better damage, lighter weight, and can harm ethereal enemies.
It is possible, though difficult, to use area of effect spells to move the armor toward you. It can be picked up though the invisible barriers. Telekinesis cannot be used to obtain the armor.
Though Martin states that "the gods have no artifacts", several artifacts apparently created by the Aedra appear in the Elder Scrolls series. In Oblivion, items like the Brush of Truepaint and Crusader's Relics are technically Aedric artifacts.

Bugs
 
 
 You can fix this by going to a save file from before you kill all four ghosts and entering the console command  . The missing Undead Blade will appear - kill him and he will go where he is supposed to.
 

Journal Entries