Blood of the Divines
When you give Martin your daedric artifact, he'll tell you that he has figured out the second ingredient needed to open the way to Cameron's Paradise: the blood of a divine. Unlike the Daedra Lords, however, summoning one of the Nine and asking for some blood is not going to happen. But Tiber Septim, who became the divine Talos, may have left some of his blood behind during his mortal years and sends you off to talk to Jauffre about it.
It's pretty easy to get side-tracked from this quest since it's activated at the same time as the Bruma Gate quest. I've put it with the "Blood of the Daedra" and "Great Welkynd Stone" quests since they are the three quests where you need to retrieve items for Martin to open the way to Cameron's Paradise but it does run at the same time as the "Bruma Gate" and "Allies for Bruma" quests, so the order in which you complete them really doesn't matter. When you talk to Jauffre, make sure that you follow up on the "armor of Tiber Septim" thread to activate this quest.
Jauffre confirms that Tiber Septim gave his bloody armor to the Blades after his victory at Sancre Tor, which became a shrine for Blades pilgrims. Then a great darkness descended on the shrine and no one has been to Sancre Tor in years. Well, that's not quite true - no one has returned from Sancre Tor in years; Sancre Tor claimed the lives of the four greatest Blades of Tiber Septim's day (Alain, Valdemarr, Rielus and Casnar) and no one has gone there since. Jauffre will give you the key to the shrine, but fears that he's sending you to your death. The darkness is the result of a curse from the Underking (Daggerfall players will remember him as Tiber Septim's battlemage, Zurin Arctus), but you won't get that info from Jauffre. At any rate, the Underking is long gone, banished to whatever realm of the hereafter might be appropriate when the Totem of Tiber Septim was used to activate the Numidium many years back. However, some of his handiwork is still around, including the curse on Sancre Tor.
You'll encounter skeletons and such on the surface of Sancre Tor and it's a relatively large area, so finding the entrance might take some time. Make sure that you have a weapon capable of damaging the more powerful undeads (Silver or better) before you go. There is one in the first section of the Sancre Tor shrine, but it's a basic enchanted dagger and not the best tool for what you need to do.
Once you get inside the shrine, you'll need to find and defeat the four Blades (who are now undead Skeletons) in order to unlock the barrier that's keeping you from Tiber Septim's armor. The first of these undeads (Rielus) can be found in the Sancre Tor section. After you defeat him, speak to his ghost to get the story of what happened and what you need to do. Then collect Rielus' Amulet of the Ansei (leveled Fortify Block effect) and move on to the next section, the "Entry Hall".
The "Entry Hall" is kind of a hub. Although there are a few undeads to fight here, you won't find any of the three remaining Blades in this section. You will, however, find entrances to the rest of Sancre Tor: the "Prison," the "Catacombs," the "Hall of Judgement," and the "Tomb of the Reman Emperors." The entrance to the Catacombs is not easy to find as it's down a tunnel that's hard to see, but you can get to the Catacombs from the Hall of Judgement, too.
The Prison level is going to basically require that you work your way in and back out again. If you've taken care of everything in your way as you go in, then getting back out again is just going to involve a bit of walking. The only oddball thing of note in this section is Warden Kastav, who you'll encounter shortly after entering. The locked door behind him can either be opened with the Warden's Key (which Warden Kastav has) or by picking the lock. After defeating Valdemar, talk to his ghost and then retrieve Valdemar's Shield (leveled Reflect Damage effect) from the skeleton. Or grab the loot and then talk - your choice.
While you can follow the same process in the Hall of Judgement (work your way in and then walk back out), I generally prefer to use the Hall of Judgement as the path to get into the Catacombs. You enter in the southeast corner from the Entry Hall and the exit to the Catacombs will be in the southern corner of the area (just go left and keep to the left from the T-intersection in front of you). The undead Blade you're looking for is in the northern part of the area. Defeat him, listen to his speech and collect Mishaxhi's Cleaver (Claymore with a leveled Disintegrate Armor effect). Mishaxhi, for those who are keeping track, is the undead Akaviri commander you need to defeat at Pale Pass in the "Lifting the Vale" quest.
The Catacombs is about as linear as you can get. You're either entering from the Entry Hall at the southern end of the area or you're entering from the Hall of Judgement at the eastern end of the area. The reason I prefer to come in from the Hall of Judgment is that the undead Blade will be right there when I enter, so the rest of the area is just collecting loot as I work my way back to the Entry Hall. Alain is not happy to see you (even after he stops being a skeleton), but you can still collect his Northwind katana (leveled Frost Damage effect).
After you have defeated all four of the undead Blades, head for the Tomb of the Reman Emperors. The four Blades will lower the barrier which is blocking your way to Timber Septim's armor. After you have collected the armor, speak to each of the Blades to send them on their way to the hereafter.
Although you can wear Tiber Septim's armor (it's an unenchanted Heavy Armor breastplate), you'll have to give it to Martin to complete the quest.