I use it at 46 and the mana cost is a bit hefty so I do not use it on all pulls. When fighting in Gulf of Gunthak it is nice to use on the troublesome shaman trolls. The ATK portion seems to go easier on the pet. I like this spell.
Personally, i think this spell SUCKS ! and i wont use it again. Your insidious retrogression has worn off. (after 3 seconds) Your instill has worn off (after 1 second). ... has fallen to the ground. (FD fail) Loading please wait ....
Just use Cascading Darkness to snare. When it sticks, you can count on it to last at least 1 minute 36.
The vendor is in the "hidden side" of echo caverns. As soon as you zone in from Shadow Haven, turn right and click on the secret wall. Horoal Deathdeal is in the hut in the northeast corner of the area, and sells it for under 3pp.