The Mark of Patience is obtained in the Tower of Riga, on the top floor(level 8). It can be a piece of cake, or it can be a nightmare.
The tower's layout changes with every reboot. Having a rogue or ranger to track you to the top floor makes it easier, but can not be counted on.
Preparation
Light
Much of the tower is dark, meaning you either need some sort of light source(lanterns, torches, war banners, gryphon feathers, etc), or sanctuary. For those who do not have the Sanctuary spell, blue orbs from salesmen in Rune are a quick and easy source, though it does have a limited duration. A Necronomicon will also work, if you have been fortunate enough to acquire one.
Gear
Most pre-level 60 gear is fraggable. This means that many of the traps in the Tower of Riga will destroy gear. If an item does not have the attribute 'insulated', it is best not to wear it in the Tower of Riga. You WILL lose it if you spend much time there. Gear with +strength is also very helpful so you can carry more tourmalines and do more damage.
Bags
Along the same lines, most bags protect anything you are carrying that might frag. Blue orbs, black tourmalines, gear that you decided you need or didn't want to store, should all go inside bags. Gnomish or embroidered bags are the ones you are most likely to find at this point in the game. They also allow you to carry more items, since there is a limited number of items you can keep in your actual inventory.
Getting There
The only way to get to the Tower of Riga is to pay 25000 gold to 'A recently-retired adventurer'. There are four of them:
Giving the adventurer 25000 gold will transport you to the Verity Isle. Recalling will take you to the entrance of Tower of Riga. Keep in mind that you will also need 10000 gold to leave the Tower of Riga again.
2018 Edit : Any level can there when appropriate world gate opens.
Getting to the top
A few north of the entrance, there is a fountain and a druid shopkeeper. He sells a few useful items(waterskins and food rations if you expect to be a while, lanterns and lamp oil if you forgot lights).
Continue north to enter the Tower of Riga itself.
There are at least two doors leading up on each level. There is a key to locked doors on some mobs. Lockpicking always works. Bash works sometimes, but sometimes not.
There are numerous dizzy rooms as well. Always try to open up in dizzy rooms to see if there is an up in the room.
If you are a rogue or ranger, or have one formed with you, tracking a djinn will bring you to level 8. An Ancient Ariel itself can not be tracked, so you have to find it on your own once you are at the top. Level 8 is technically the top floor, but the path to the Ancient Ariel is also an up.
An Ancient Ariel encounter
The first up on level 8 is a no-form room. It will break up a form as soon as you enter the room. An Ancient Ariel is in the next room up. He is not exceptionally difficult, but you will have to flee. Flee if he casts. Be warned that he can transport you back to the bottom of the tower, and often will if you already have the mark. When you do flee, make sure to go down again immediately before you rest. The down immediate to the Ariel seems to randomly teleport you back to the bottom level of the tower, which you don't want to happen if you're here for the mark.
Bash/flee, backstab/flee, and cast/flee are all standard. If you have none of these (Paladin/ValkyrieAn Ancient Ariel/Valkyrie" class="wikipageedit">?), just be careful. Sanctuary is recommended. He doesn't take long to kill. When he dies, he will often drop hematite and/or astral reagents. If you do not already have the Mark of Patience, continue up to Cassandra and you shall receive it.
Getting Back
To leave the Tower of Riga, simply recall and go north to the druid shopkeeper. Give him 10000 gold, and he will transport you to Sigil, regardless of which town you were in to begin with.
Also, if you have your recall set to anything other then the nearest point of recall, such as the The Rune Orphanage. Recall will take you there, saving you the 10,000 gold.