Using Envenom as an AoE finisher
Envenom only deals damage to a single target, and so it might seem that it's not an AoE finisher. However, we have to always be looking at the Deadly Poison effect. Envenom's buff increases our chance to apply Deadly Poison to any of our targets, including those we're hitting with AoE attacks. Deadly Poison has a base proc rate of 30%, and Envenom gives us a 15% bonus to that chance. The net effect is that our Deadly Poison proc rate increases by 50% (from 30% to 45%) during Envenom. Our Deadly Poison procs are typically our top source of damage in AoE situations.
Ideally, we want to pool our energy, use an Envenom to gain the buff, and then spam Fan of Knives for the duration to gain the extra poison procs. We want to cram as many FoK attacks in during that Envenom window as we can. Even though it's a single-target ability, Envenom actually scales with the number of targets we're attacking. When there are more targets nearby, we see more FoK hits, which means we get more Deadly Poison procs when under the Envenom buff.
Zero ramp-up time
Once we start working Fan of Knives into our rotation, all of our targets are going to be coated in Deadly Poison quickly. We always want to be in melee range with Slice and Dice active and attacking one of the targets with our regular melee attacks. We can tab-target between enemies right after we use a finisher, since there's no ramp-up time when attacking a new target. We can use this to pick out certain mobs that need to die quickly or to interrupt a caster mob that's in a pack.
Rupture works against a few targets
Rupture itself makes up less than 5% of our overall damage, but its secondary effects like Venomous Wounds and Venomous Vim are worth the price of admission. We can use Fan of Knives to generate combo points for us, while using tab-targeting to spread Rupture to multiple targets. With Rupture ticking away on a couple of enemies, we'll be getting more energy to spam more FoKs, and dealing good damage via Venomous Wounds. Rupture itself isn't dealing the damage, it's the secondary effects we want.
Rupture is great if there are only a few targets nearby. For example, Rupture cleaving is quite common when facing the Stone Guard as assassination. The problem is that Rupture cleaving doesn't scale well with a large number of targets, as you simply don't have the energy or combo points to get Rupture up on all of them. We always want to keep Rupture rolling on at least a single target, because the opportunity cost is low enough to make it worthwhile.
We can keep Rupture up on 2-3 targets using 3- or 4-point Ruptures without too much issue. If there are more than 3 targets around, it's not worth trying to stack Rupture on any additional targets. Many rogues are currently focusing on only maintaining a couple of Ruptures and then focusing on the rest of the AoE rotation. Trying to stack low-point Ruptures on a plethora of targets simply doesn't work out in the end.
Crimson Tempest is lackluster
Crimson Tempest might be a great finisher for subtlety rogues, but it doesn't really have a designated place in assassination's toolbox. We will almost always prefer using Rupture or Envenom in AoE situations. I only find myself using Crimson Tempest during Shadow Blades, as it's possible to have Rupture and Envenom both rolling and yet to still find yourself with a glut of combo points. Crimson Tempest also isn't too bad if your targets will be dying in less than 10 seconds.
Crimson Tempest can proc our poisons, but only once per target. An Envenom followed by a FoK or two will easily generate more Deadly Poison procs and more damage. We have to focus on Deadly Poison in order to maximize our damage, and Crimson Tempest isn't a very effective vehicle for Deadly Poison. Even though we might be spamming Fan of Knives for an entire encounter, it contributes less than 5% of our overall damage. FoK is simply a vehicle for applying Deadly Poison to our targets, just like Envenom. Envenom's dominance over Crimson Tempest will only continue in patch 5.2, when our new set bonus improves Envenom's effectiveness and doesn't affect Crimson Tempest at all.
Sneak in every Wednesday for our Mogu'shan Vaults guide, a deep-dive into the world of rogue rotations -- and of course, all the basics in our guide to a raid-ready rogue.