The Forest of Murthrid
To those who dwell in Leng, there is little doubt - along with the Uncharted Lands, the Plains of Dust and Blackreach - that Murthrid is one of the most dangerous wildernesses in the entirety of Leng. A vast and seemingly endless forest that sprawls across the eastern tip of the continent, the sheer size of it staggers those who see it for the first time, occupying the whole of the horizon as one looks east. Yet the Murthrid that lives today is a shadow of the great and trackless forest that once covered almost all of Leng - and it holds the ancient and storied mysteries that have slept there since the dawn of time.
The Wood Elves
Murthrid is known throughout Leng as the home of the wood elves, but in truth they are a relatively minor feature of the forest. Although the forest is size of Vingaard or even bigger, the elven population is only about 30,000 or so - almost a hundred times less populous. In truth, the wood elves are like scattered leaves throughout the forest. Most elves live alone in the forest, hunting and subsisting for themselves and living a nomadic life. They are always at least level 1 rangers; many interact only rarely with other elves. Instead of being a member of a town or a community, as a human would, the more nomadic elves are more likely to have a few old friends that they may not see for a decade at a time, and otherwise return to elven society only when they need to.
Of course, there are safe places in Murthrid where elves can congregate - many of them, in fact. But finding your way between them is difficult if you do not know the trick. The wood elves gather in places called aelin, which translates in the Common tongue as "steading" or "homestead". These are places that are protected - whether by natural circumstances or the protective magics of a druid, mage or fey - and they are usually within easy reach of food and fresh water. Some of them are strikingly ostentatious, while of others it is said that one could pass within ten feet without knowing. Some contain secret caverns or tall towers - others are simply lush oases in the forest where one may rest at ease.
The steadings of the wood elves are not distributed at random, either - they form tenuous chains and links through the safer parts of the forest that constitute elvish territory, like an invisible network of roads. These roads can be followed only by those who know to look for the subtle cues, however. A bent branch pinned back might indicate that one should turn left, or a small pile of pebbles may warn that a steep climb is ahead. Elves native to the forest can follow these cues without error, and can do it so naturally that they simply seem to know exactly where they are going at all times. Rangers may also recognise the signs, and can follow them as long as they make a successful tracking check each day.
The elvenpaths do not just connect individual steadings of scattered individuals, however - they also lead one deeper into Murthrid, into the heart of elven territory. If one follows them for long enough, they will eventually find themselves in the more populous centers of wood elven culture. Deep in the heart of the forest, there are places where elves gather in large numbers - such asthe great communities that congregate around the Druidic Circles, the ancient shining cities from the days when the wood elves still built cities, and the fabled Towers of Wisdom, where the collected arcana of millennia is kept in safety.
Travelling Outside the Elvenpaths
As noted above, wood elves can easily navigate the forests by following the elvenpaths. For those who do not know the elvenpaths, however, or for those that wish to brave the deep forests, there is no such guidance available. The areas outside of the elvenpaths are inhabited by solitary elven wanderers and nomads; many of these have spent their whole life in their "territory", and know it like the back of their hand. If you only intend to travel within a few miles of the paths, a local guide may be enough to keep you from getting lost.
Travelling into the truly deep and forbidding parts of the forest, however, is a daunting task. There are no maps of the forest, and even if you know the approximate direction and location to which you are travelling, it is almost a certainty that you will become lost eventually. Unless you are following a specific trail (as with the Tracking NWP) or following very specific directions, the base chance per day of becoming lost - as per the DMG - is 70%. This chance can be improved or worsened by the circumstantial modifiers found on Table 82 of the DMG.
Murthrid is vast, trackless and very dangerous, and most who become lost in the forests are never seen again. Climbing a tree will reveal little more than endless forest stretching in every direction, unless you are lucky enough to be near a major landmark such as one of the Towers of Wisdom. Even the ability to take wing as a bird and fly would avail you little. Characters may have magical means of finding their way back to civilisation; or, if they are lucky, they might happen across one of the more friendly denizens of the forest who may be inclined to help them. Otherwise, they have little choice but to wander aimlessly in the hope that they find their way back to civilisation before one of the native denizens of the forest slays them. It is no surprise, then, that the elves do not feel the need to defend the vast border of their domain; the forest itself is a more effective deterrent than any military force.
The Towers of Wisdom
Of all the places in Murthrid where wood elves gather, the Towers of Wisdom are the most famed. In ancient days, when manking and the halflings were little more than tribal children making war with sticks, it was the power and wisdom of the elves that elevated them. The elves taught them metallurgy, woodcraft, alchemy... and sorcery. The great and powerful elves that created the Dragon Orbs in ancient times may be gone now - dead or long since departed on the Swan Ships - but even so, the reputation of elven sorcery is a great one.
The elves guard their knowledge jealously. The Towers of Wisdom are protected by a force greater than any city wall - the impassable forests of Murthrid, which no army could ever hope to brave. They are deep in the forests, the nearest of them requiring two weeks of travel on the elvenpaths to find. And even amongst elves, the Towers of Wisdom are difficult to enter. Although humans may believe that all elves are united by their vague and unknowable motives, the elves of Murthrid have lived long enough to know that there are many different pulls and intrigues, especially amongst their own kind. The mages of the Tower of Wisdom - the Adonessai, or Peacekeepers - are bound by powerful oaths that constrain them more tightly than any geas.
More information about the Adonessai can be found here.