First of all, let's look at how much mana the talent gives us with a simple once-every-four minutes instead of once-every-five minutes analysis. Shadowfiend restores 30% of your maximum mana. In my case, that's 36000. 36000 mana every five minutes is 120 mana per second. 36000 mana every four minutes is 150 mana per second. So you get 30 mana per second for the two talent points. I haven't made a post about benchmarking talents yet, but this is clearly not that strong.
The real advantage of reducing the cooldown is not that it gives you 30 more mana per second if you cast it on the cooldown every time. It's that it allows you to get two shadowfiends in a fight where you would normally get one. If a fight is five minutes and you get to cast a second shadowfiend because of the talent, then that's 36000 more mana for the fight, or 120 mana per second over the fight. The question is, will Veiled Shadows actually do that for us?
I'll make this easy on myself to get a ballpark figure, and then talk about the complexities. First of all, I can ignore mana regeneration from any source that regenerates mana proportional to the length of the fight. So ignore spirit regen, Rapture, the Mp5 buff and Replenishment. When I talk about how much mana I am spending, I will be talking about how much I am spending above that continuous mana regen. I'm also going to assume I'm spending mana at a continuous rate.
If I get to Hymn of Hope once during the fight, then that will restore 9.2% of our base mana1 and a mana potion is around another 8.3% of my max mana. So I have around 114.5% of my base mana pool to work with. Just because I'd rather work out of 100%, I'll instead say that shadowfiend restores 26.2% of my augmented mana pool.
I wouldn't want to cast shadowfiend as soon as the fight starts because most of the mana would be wasted. The earliest you would want to cast shadowfiend is:
Where mana_spent is measured in percentage of augmented mana per second.
Veiled Shadows will give me an extra shadowfiend cast is the length of the fight minus t if between 240 and 300 seconds. We probably want to ignore at least those cases where the second shadowfiend doesn't last its full duration, so look at cases between 255 and 315 seconds instead of those between 240 and 300. Of course getting an extra cast isn't the only thing to consider. If Veiled Shadows is giving me an extra cast, am I actually going to use that mana? On the flip side of this question, am I going to have any mana left when it comes time to cast shadowfiend again? If this model has me running out of mana at the three minute mark, then it isn't a model of a winnable fight and Veiled Shadows is irrelevant.
So the amount of mana I have left when it's time to cast my second shadowfiend is:
And if we substitute in the above equation for t this simplifies to:
Which is pretty easy to work with. Our conditions above were that the second shadowfiend matters, so the amount of mana we have left should be less than the amount of mana we are expecting to spend in the rest of the fight, and also that we should actually have enough mana to get to the second shadowfiend without running out, so the amount of mana we have left is greater than or equal to zero.
Simplifying these two requirements, we get the following:
Substituting one into the other, we arrive at the conclusion that the length of the fight must be greater than 303 seconds in order for the second shadowfiend to be helping us. Basically in a five minute fight there is no way for the second shadowfiend to work out, and there are a lot of five minute fights. In the event that you are spending almost exactly 0.416% of your augmented mana per second, this talent would probably be very good, and maybe even necessary, but possibly not enough to make winning the fight feasible with your current mana.
Of course going over 303 seconds is no guarantee that the talent is working for you. We need it to not only be the case that a four minute shadowfiend is working for us, but also that a five minute shadowfiend would not be. Using similar analysis for mana expenditure per second, we can see that starting at around 379 seconds, the five minute shadowfiend starts being viable for a second use. The four minute shadowfiend won't be viable for a third use until at least ten minutes. Furthermore, the windows to take advantage of the extra shadowfiend are quite small. In order for the talent to be helping, you need to be casting your first shadowfiend between approximately 72 and 80 seconds into the fight. If you are casting it sooner - and not wasting it - then you are burning through your mana so fast that you have no chance of making it to the end of the fight. If you are casting it later then you are using mana at a slower pace and don't need the extra shadowfiend cast anyway.
The shape of damage on a given fight might alter these things. Veiled shadows might be useful on a fight like Halfus where you spend a lot of mana early on, then have a big lull. Conveniently, Halfus is also a six minute fight, which is where Veiled Shadows really has an opportunity to come into play. Use shadowfiend when you are still fighting the drakes and you might both get full effect out of it and have plenty of mana to make it to your next cast, which you might need because you are forced to heal up after the roars. This kind of high damage at the start, high damage at the end, lull in the middle fight, however, is pretty rare, and it is also a pushover that you don't need a special talent spec for.
Overall, I think I am just not hitting the window to make this talent work for me. The question is, where to put the other two points. For now I think I'll stick them back in Surge of Light, but with patch 4.0.6, both Improved Renew and Soul Warding present themselves as reasonable alternatives.
1. Hymn of hope gives 2% mana for each of four ticks. However, it also increases maximum mana by 15% for 12 seconds. Assuming you get all four ticks, that means that the second through fourth tick give 2.3% of your normal maximum mana, since your maximum mana will be higher, so it gives 8.9% of maximum mana. But also you have increased maximum mana for a total of 18 seconds from when the first tick hits you to 12 seconds after the last tick. That means 18 seconds of getting 15% more replenishment, which is 0.1% mana per second, which means 0.27% more mana maximum mana.