Some abilities that have no effect in battle you have to buy, they are just for show and most people call them "fun abilities" you buy it from your Ranger Trainer, i think he is in willow wood if your a Qeynos guy, no idea if your from kethlin.
It is a varient on the snare line available to people who purchase the splitpaw adventure pack. It has a root effect in addition to the standard snare. It is definitely better than the standard line CA.
The adventure pack spell lines are only available as recipes purchased within the adventure pack. The recipes can then be made and the resulting spells sold by players. Check the broker for an adept 3 version of the spell.
I have purchased the AD3 versions of the priest spells for my warden and fury and get quite a bit of mileage out of them.