This can only be done after a reboot because the key you need only repops after a reboot. Also note, that only levels 61-119 can complete this run.)

Kill the Silver Knights in Sigil castle to obtain as many delicate jewelry boxes as you can get. Drop and sac them until you find one or more that tell you to remove the items inside first. With these boxes in your inventory you will need to fight something that can either use breath attacks and/or cast spells that can frag items. (*Note, that when doing this it's not suggested to wear items that aren't insulated as any equipment you are using that isn't insulated may also be destroyed.) When the delicate jewelry box(es) are destroyed you should find a silver unicorn key in your inventory, one key per box. (This key is insulated so you need not worry about it being destroyed when you are fragging the delicate jewelry box(es).

Once you have the silver unicorn key you'll need to wait until 5:30 AM game time and then go to the Gallery in Sigil castle. From the northeast corner of the Gallery chamber go one room south and search to reveal a locked exit east. Unlock the east exit and enter. (This room is a private room so only one person at a time will be able to enter it.) The guard there should allow you to go down if you enter the room around 5:30 AM, at other times he blocks the way.

When you are at the bottom of the Concealed Stairway you will want to prepare yourself for a tough fight. When you are ready go west into a Dank Cellar where you should find Novrar the necromantic beastmaster. This mob is a Satyr that hits very hard for a second classer to handle solo. (Novrar can't be blinded with flash bombs.) He is extremely aggro and uses necromantic spells and his spear does added damage and increases his healing rate as well. (A Valkyrie/Druid combination appears to be one of the better class paths to use if you want to solo this mob. If you are druid you will want to make use of your ash invocation spell to reduce the damage taken from his necromantic spells. Also note that it hasn't been determined whether the bottom of the Concealed Stairway is a private or solitary room so it may be possible to bring more than one person down to fight Novrar if you enter one by one.) When Novrar dies you can loot the following possible items: a bonedriven shield, a spear of the hemic unicorn, and an equine femur.