So I found myself with my level 71 Warlock at the end of the quest chain and since I had just respecced to Demonology, I felt cocky enough to attempt to solo this quest. This is still a 3-man quest so I wasn't 100% sure I could do it but at the same time I didn't want to wait who knows how long until I could find someone else.
The last time I finished this quest was on my level 71 Rogue with help from a level 80 Rogue that was nice enough to give me a hand. Unfortunately, even though I survived to finish the quest, the 80 Rogue died. He had forgotten how this fight works and had basically killed himself. More on this in a moment.
Up until a couple of days ago, I had played my Warlock from 1 to 71 as Affliction, and I loved it. On a whim, I decided to get Dual Spec and make my other spec Demonology. Here's my Demo spec, the one I had when I attempted Last Rites, along with the glyphs I'm using at this level:
Before I started the fight, I buffed up with some Shoveltusk Steak for Stamina and Spell Power, a Scroll of Stamina V and I also used another scroll on my Felguard. We both had over 10K HP. Now, I am aware that right from the start I made a bunch of mistakes during this fight with Prince Valanar.
For one thing, I totally forgot to use Immolate. An extra DoT would have been great. Instead, I used Curse of Agony and Corruption as well as Shadow Bolt.
Another mistake (I think, but not sure on this) is that I used my Felguard instead of my Voidwalker. I believe the VW would have been a better tank for this fight. Or not. At least I didn't forget to use Demonic Empowerment whenever it was up.
My third big mistake was that in the heat of the battle I totally forgot that I had Metamorphosis. To say it would have helped me would be an understatement.
My strategy here was to keep the Felguard alive and keep the DoTs up on Valanar. To this end, I punched Health Funnel when my pet was getting low on health. The big thing to watch out here for is the Prince's special attack called Soul Deflection. Whenever he starts channeling this spell, which looks like a tendril that connects to your character, you need to stop DPS or else you will be reflecting all the damage you do back at yourself. That, in fact, was the reason why the 80 Rogue died.
I'm not exactly sure how Soul Deflection works with DoTs but I believe each tick also damages you because my health went down alarmingly several times. Each time he started channeling that spell, I would recall my Felguard. During the fight I had to use some healing unfortunately: a Healthstone, a health pot and my Herbalism ability Lifeblood.
Finally, the Prince dropped. At the end, I was left with about 20% life and the same in mana. I have to say I'm pretty happy with how I handled this fight. Although there were a few mistakes that made the fight harder, in the end I prevailed. Still, this doesn't mean I am very good at Demonology, in fact it's the opposite.
From the rewards I chose the Staff of the Purposeful Mendicant.