EU/Horde (active), Alliance(Idle)/Twisting Nether, Allonsus/Casual Noob(ish)/Celdor#9837 (Discord)
Characters
My main is Druid Feral, Celdora. I love druids so much that I ended up having three -.- I also gear other alts for SL: Rogue (assa), Demon Hunter, Hunter (MM & Survival), and considering Warrior but it is still in a idle due time issues.
Topic
My post is about Horrific Visions (HV). I have only recently completed 5 mask run in both Ogrimmar and Stormwind. I'd like to give some comments on what helped me to achieve that.
Things to consider
There are a couple of things I'd like to add:
Bosses and minibosses
There are only a few bosses that caused me troubles.
Voidcrazed Hulk. The lost area in Valley of Wisdom. This boss always drains sanity unless I miss something. Approach it with the Gift of the Titans on. Use the trick I mentioned before.
Raxxar. The lost area in Valley Of Honors. Probably the hardest boss in Ogrimmar. Try to approach him with an extra Orb and use it right before the fight. Also, make sure the Gift of the Titans is about to get ready (something like 10 secs of remaining CD) because killing a Void Roars will trigger it. Common thing is to use Blood/Drums/Time when Gift triggers and use one of the two strategies. Either focus burst damage on the boss (might be hard with 5 masks) and nuke him or kill single Roars sequentially. Use CC if possible and interrupt their casts. Save AoE essences when a number of Roads engage. This is what I've done. After killing pets, Raxxar is just a peanut.
The Boundless Corruption. The lost area in The Mage Quarter. It's considered to be one of the worst of all mini-bosses. I always fight him moving around to avoid blue patches. You have to avoid its breath. Move slightly away when you see Madness: Split Personality is about to occur to make more room to escape.
Overlord Mathias Shaw. The lost area in Old Town. This boss is tricky but not that hard. I am only mentioning this boss here because this fight is more about positioning, rather than the damage itself. With 5 masks and two kinds of madness on, you need to be careful:
Alleria Windrunner. The main boss in Stormwind. The difficulty is to use the right strategy and expect hat's coming. It will be almost always a fail without practice. Having said that she is fairly doable when you know what you're doing. My advice:
Difficult packs
I find difficult packs in both scenarios. The first is in The Mage Quarter on the way to the second mini-boss. There are two casters and one big Void minion. For some reason, when I am not careful I end up the instance right here. I suggest you have the Gift ready, or at least have ready potions, and/or cooldowns, or use tank spec.
The second "difficult" pack is in Valley of Honors right before Raxxar. Similarly to HV of ST, if I am not careful, I end up the instance right there. Again, approach the pack with ready potions, cooldowns etc. because the damage is much more devastating. It might be different for other players, though.
The third pack, which you can avoid is next to the entrance to the Valley of Wisdom at the lower ground next to the ramp. There are a Faceless and two casters very close to another pack next to the water pool with a caster and two mobs that can stun. Essentially, if you cannot kill those quickly and if approached without tactics those two packs will end you.
Most dangerous mobs
In Ogrimmar, a general rule is to always kill casters or CC them first. They are most dangerous because of incapacitating a player and draining life. In the Valley of Honor, bugs are also tricky. I usually CC the big ones first and kill the rest. Be careful as they can stun.
There is also a number of hidden bugs. Two on a bridge, one next to two a big pack in a corner right before the last mini-boss on the way to Rexxar (the pack without the big bug), one with that mini-boss and three with the last pack. I consider them dangerous when they all engage together. I don't pull these packs directly, I always use either small steps to reveal small bugs and then engage the rest or use range spells to pull the pack first and then kill the hidden ones.
In Stormwind, kill casters first or CC them. In Old Towns, there is a number of hidden agents. Just make sure you don't pull everything.
Hope this post can help :)