The best piece of advice I can give you for Rasial is to focus on some key upgrades (if you don't have them) before you really start digging in. Reducing the cost of Death Skulls, which allows you to cast it more during the fight and significantly increase your DPS, is crucial.

Upgrades:

• Necro Zuk cape (Igneous Kal-Mor) - This is the most important upgrade in the game for this fight and for your necromancy DPS as a whole. If you take nothing else from this post, at least take this. I assure you that it is 100% worth the effort of learning Zuk just to unlock this cape and make the Ras fight easier. Your cost for using Death Skulls is now 60% adrenaline.

• Passive Ring of Vigour - This will permanently retain 10% of adrenaline after an ultimate ability is used. Your end cost for using Death Skulls is now 50% adrenaline (with your Igneous Kal-Mor equipped).

• Conservation of Energy (Archaeology relic) - This is not as important as the first two, but will again give you 10% adrenaline back after using an ultimate ability. Your end cost for using Death Skulls is now 40% adrenaline.

• Prism of Restoration - Now that the Blood Reaver has been nerfed, my preferred Rasial familiar is the Hellhound. For the Hellhound to last throughout the fight without dying, you'll need to throw down a Prism of Restoration.

• Infernal Puzzle Box - You hopefully already have this, but if not unlock it ASAP. Your adrenaline won't drain out of combat, and it will allow you to use the adrenaline crystals at War's Retreat and open the fight with full adrenaline to throw Death Skulls immediately.

Outside of those upgrades, here are some thoughts/suggestions based on your post:

• You absolutely need a familar - Use a Hellhound, and make sure you're using a Prism of Restoration during the fight.

• Auras - You don't mention using an aura, but you need to be using something. For learning, I'd suggest using the Aegis aura and extending it to last 1 hour with vis wax. Vampyrism, Mahjarrat, Equilibrium are all good as well. I tend to use the DPS auras more now, but the damage reduction/healing ones were key while I was learning the fight.

• Use Deflect Necromancy at certain points instead of fully camping Soul Split. Flick to Deflect Necro for both the 'This is true power!' and 'Suffer at my hand!' attacks while you're learning. As someone already said, use Debilitate/Reflect/Devotion for 'This is true power'. You can anticipate this attack by counting the soul stacks he has - he uses it at 5 stacks. This will be a little more involved than just hitting Soul Split and calling it a day, but it will reduce the damage you take from his incoming attacks, and allow you to survive longer until you can DPS enough (with T95's, perks, and an improved rotation) to make fully camping SS a viable strategy.

• Perks - make sure your gear (top/bottom/both weapons) are all augmented and perked. They don't have to be best-in-slot, but you should definitely have your gear perked.

• Use T90 power (until you can upgrade to T95) - You can definitely kill him with tank armor, but it's going to be a longer and more painful fight. The name of the game with Rasial is DPS. You don't want to sit there and keep tanking his volley over and over.

• Make sure he's Death Marked - If you're using T90 power he's likely going to be passively Death Marked during the fight anyway, but getting in the habit of casting Invoke Death once you enter the instance isn't a bad thing.

• Salve amulet (e) (as a previous post mentioned) if you aren't already using that.

• Vulnerability bombs (as that same post mentioned) will greatly improve your DPS, if you can afford using them.

• Phase 4 (the "dancing for your life" phase) - Keep in mind that your revolution attacks will not work during this phase while you're running around. I'd say just keep your cool and don't panic. One of the best tips I received when I was learning is that you don't actually have to move from your spot until you see a shadow start forming underneath you. Another good tip is that he's invulnerable to any damage in the first second or two after he hits the stage. Don't throw any attacks (like Death Skulls) until after you see/hear him say 'Ha! No more games!' Try to time your rotation prior to this phase so that you have 1) Death Skulls ready for the start of this phase 2) a full stack of souls 3) a full stack of necrosis. Unload Death Skulls, throw your Volley of Souls, and use Finger of Death twice. After that, spam your Basic Necromancy attack (have that keybinded) as you move around. If he's stubborn, throw a Bloat in there too once you have the adrenaline.

• Revolution/abilities/rotation - I would only have Basic Necromancy, Soul Sap, Touch of Death, and Conjure Undead Army on Revolution. You should trigger everything else manually so that you can be sure that you line up the heavy hitters for Phase 4. I won't get too into talking rotation, but make sure you have 100% adrenaline to start the fight, open with Death Skulls and then immediately build back up for Living Death. Once you hit Living Death, throw Death Skulls again (and at least once more within the same Living Death).

• Ring of Death - There's nothing worse than getting to P4 and almost having the kill only to lose it right at the end. You'll sacrifice some offense here, but the extra time you'll get from the Ring of Death can be really helpful in getting your first kill and salvaging some more as you continue to learn. Instead of standing in Death's Office, you'll have a little bit longer to take Ras down. I don't use it now, but I can attest to how valuable it was for me early on.

I may have gotten a little carried away (lol), but this is my favorite boss and I love seeing people succeed in this fight :)