I have heard of your many deeds of skill and bravery. I feel I can trust you with something I have been hesitant to speak of...
Drognan and I have concluded that the Dark Wanderer who passed through here recently was Diablo, himself! Drognan believes that Diablo is searching the desert for the secret tomb where the great Horadric mage, Tal Rasha, keeps Baal imprisoned.
You must find Diablo and put an end to the terrible evil that has fallen upon our city. Drognan is wise and is sure to have some helpful advice for you as to how Tal Rasha's tomb may be found.
It may take you quite some time to find The Tomb. May you be ready when you do.
- Quest log will also show symbol of true Tal Rasha's Tomb, so it can be found among seven tombs whose entrances are found along northwest and northeast walls of Canyon (previously, symbol of true tomb could only by deduced by identifying missing symbol from six floating above Horazon's Journal platform).
Position of symbols around edge of waypoint indicates direction to travel to find corresponding tomb.  
- Upon entering true tomb, search for chamber containing orifice. Entrance is in southwest or southeast wall, so it is approached by traveling northeast or northwest respectively (although it may be approached by traveling southwest, then turning northwest).
- Components of Horadric Staff and Horadric Cube needed to restore it should have been collected during the mandatory completion of the Horadric Staff: if it has not already been restored, do so now and place it in orifice to finally complete earlier quest and gain access to Tal Rasha's Chamber.
- Before entering, it is advisable to cast Town Portal and make sure character is prepared for battle...
- Although he is alone, character is trapped in a small chamber with him and can only escape by casting Town Portal: overcoming him can be one of the greatest challenges faced in Normal, particularly for fragile ranged attackers and casters.
- Aside from a standard Attack, he uses Jab and Smite (which requires a hit-check and can be blocked), the former of which is particularly dangerous in Normal when cold damage added by Holy Freeze is significant for that level (Cold Resist can drastically reduce danger: if nothing else, use Thawing Potions). Fortunately he has not used Charge since patch 1.09, at the very least. 
- Since all his active attacks are melee and apply physical damage, returning damage using Thorns, Iron Maiden, Iron Golem thorns or Spirit of Barbs can be a useful strategy (at least in Normal, where he is most dangerous), particularly if target is mercenary (to which he applies 200% damage) or pet (to which he applies 400% damage).
- Once he is defeated, exit in northwest wall opens and allows remainder of Tal Rasha's Chamber to be explored. Travel northeast to find and talk to Tyrael.
I thank you, mortal, for my freedom. But I did expect you earlier.
I am the Archangel Tyrael. I came here to prevent Diablo from freeing his brother, Baal. But I have failed. Now, Terror and Destruction roam free throughout your world.
Even now, they head towards the Eastern capital of Kurast - to the very heart of the Zakarum Temple. There they hope to find their eldest brother, Mephisto, the Lord of Hatred who was imprisoned there ages ago.
If the three Prime Evils unite, they will be invincible. Though it is unclear as to what their aims are, it is certain that they must be stopped at all costs.
I am broken and the energies that tie me to this world are diminishing rapidly.
You must take up this quest and prevent the Three Brothers from reuniting. You must cross the sea and search for Diablo and Baal in Kurast.
Now hurry, mortal... Time is running out for all of us!
After talking to Tyrael, he opens a portal back to Lut Gholein
You have done very well. Few could have come this far, let alone discover the True Tomb of Tal Rasha. Unfortunately, I hear that Diablo and Baal have eluded your grasp. This is most unfortunate...
If you wish to travel East, I have authorized Meshif to give you passage by sea. I imagine he should be very anxious to leave by now.
Good luck on your quest, and thank you again for saving my beloved city. You will always be welcome in Lut Gholein, my friend.
Jerhyn tells me I should take you east to Kurast. I haven't been there for several years, but rumor has it that things are pretty grim.