Minotaurs will spawn here when you first enter the sanctum, and stepping onto some of the pressure plates (#2) will spawn them as well.
2 - Pressure Plates
You'll need to step on three of the pressure plates in the correct order to open the grates at #3.
3 - Closed Grates
If you look through the three grates here, you might see some rune images lit up on the far wall. This is a hint for how to open the grates. You must step on the pressure plates with the images of the runes on them. That is, you must step on the plates in this order: purple (#2a), orange (#2b), and then blue (#2c). That will cause the three grates to rise, allowing you access to the final pressure plate (#4).
4 - Pressure Plate
When you step on this pressure plate, the closed grate at #5 will open.
5 - Closed Grate
You'll have to solve the pressure plate puzzle (#2 and #3) and then step on the pressure plate at #4 to open this grate.
6 - Fire
You'll have to jump up to the edge of the fire to actually use it.
7 - Soul Spheres
Soul spheres are required to defeat Ikrella the Witch (see #8). Each soul sphere is guarded by a demon gazer, which is a more powerful version of the fire gazer that you might have seen before. Since you'll need to grab the soul spheres while dealing with Ikrella, you're probably best off just ignoring the gazers, and doing your best to run past them on your way to and from the soul spheres. The demon gazers won't chase you (it doesn't look like they can fit through the doorways).
8 - Confrontation
When you arrive here, you'll find three people on the platform in the center of the room: Ellowyn (imprisoned in a ward of confinement), Ikrella the Witch, and Lord Barrowgrim. Ikrella will immediately begin taunting you. Lord Barrowgrim will chastise the witch, but to no avail, and he'll quickly leave for Talendor. That means you'll have to battle the witch one-on-one, which is no easy thing.
If you've chosen Sisterhood classes for your character, then Ikrella might seem a little familiar. She's like a more powerful version of Greneuvenia the Pox (from the War Witch ascension quest). Ikrella casts a lot of nether spells, including "slow," "gripping vines," "choking cloud," and "locust swarm." If you have the gas mask from the Naga Temple, then you can ignore the locusts and the clouds, but the vines can be a major problem. Usually, they'll disappear as soon as you're hit, but sometimes they won't disappear ever, and you'll have to load your game. So be sure to save your game during the battle, and if you have crystal wards of petrification or crystals of unpetrify, so much the better.
Of course, Ikrella is no ordinary witch, and simply hacking on her won't work. Every so often while you're fighting Ikrella, she'll teleport to a different part of the room, and she'll become fully healed. Fortunately, Ellowyn will tell you how to defeat her. You'll need to take each of the four soul spheres (#7) and throw them into the fire (#6). (You'll have to jump up onto the edge of the fire to do this.) Once you've placed all four soul spheres into the fire, Ikrella will stop regenerating, and you'll be able to kill her. Once Ikrella is dead, she'll drop a fifth soul sphere, and dropping that one into the fire will free Ellowyn.
After the battle, Ellowyn will describe how she came to be imprisoned by the witch, and how, in an effort to protect Vadane, she gave her word to wed Lord Barrowgrim. She'll then ask you to travel to Fargrove and inform Lord Davenmor of what's going on. However, when you tell Ellowyn "it is the day of days" she'll hand over the Ring of Protection to you, and Emmindor will arrive. Emmindor will send Ellowyn to Fargrove, where she'll be able to deliver her message herself, and he'll send you to Talendor, where you'll finally be able to face Lord Barrowgrim.
Note: Emmindor will teleport you away right after you've spoken to him. So if there's anything left in the room that you'd like to kill (such as those demon gazers), or if you need to set up your equipment for the upcoming battle (such as putting on the Ring of Protection), then you should do that first. It's also possible Emmindor will just speak to you on his own, not giving you the chance to do anything.
Another Note: For a much easier time in the battle against Ikrella, simply go to the pressure plate at #5. There's a window there that overlooks the platform where Ikrella is standing, and you can attack her from there with ranged attacks. Ikrella won't disappear or regenerate or do anything but stand there if you attack her from the window, and she'll still drop a soul sphere when she dies. The only downside to this approach is that it will mess up the dialogue you would have seen if you had approached her normally.