"Yes - we had suspicions at least, that's why 'the greatest spiritual threat to the Empire' is verbatim wording from multiple Synod motions.
So, brief history lesson.
Before the Empire was here, most of the Empire's territory was Terunael - stretched further than the Empire, mostly concentrated in big cities, lots of Spring magic.
Then the surrounding orcs became a pressing threat, and the Terun people performed an enormous Spring ritual - and, whatever they'd intended to do, what they got was the Vallorn.
It, essentially, ate all their cities, then tried to eat everything. Their survivors came up with another ritual, to direct its power away down the Trods - which is what powers the magical vitality you get when you walk them. Dedicated walking of the Trods is gradually drawing the power out of the Vallorn. The one at Miaren was defeated during the time of the second Emperor, at great cost - and that strengthened all of the others, hence the rather long time before we have been able to even think about trying again.
Recently, the momentum behind discovering that people are stuck in there - that was a recent discovery - has meant there's an effort to put a library together in Hercynia, an unprecedented expedition into the heart of the biggest Vallorn in Broceliande, and in general things are moving, although estimates are still at least a year before all the research is put together.
Winter magic is our main solution to the spread of the Vallorn, other than the Trods - there's a curse called Wither the Seed which curses a territory for a generation, makes it hard for anything to grow there and for people and animals to conceive, but slows down the Vallorn. Liathaven and Broceliande are currently under it.
Each Vallorn has a 'vallorn heart' which is in the ruins of the old Teruneal city it ate; the expedition might be getting our first look at the main node, if enough people showed up and they are lucky.
The Vallorn exudes a poisonous miasma and contains a considerable amount of giant wildlife as well as the hijacked people, and between them they tend to give anyone who ventures in various diseases, which makes it a dangerous operating environment; the expedition is under some limited-time magical protection from the Summer Eternal of Adventure, Rhianos."