But your way through Mirkwood is dark, dangerous and difficult, Water is not easy to find there, nor food. […] nuts are about all that grows there fit for food; in there the wild things are dark, queer, and savage. […] But I doubt very much whether anything you find in Mirkwood will be wholesome to eat or to drink. There is one stream there, black and strong which crosses the path. That you should neither drink of, nor bathe in; for I have heard that it carries enchantment and a great drowsiness and forgetfulness. And in the dim shadows of that place I don't think you will shoot anything, […] without straying from the path.
–The advice from Beorn in Queer Lodgings, The Hobbit
The Mirkwood Mountains are located south of the Woodland Realm in the north-east of the Mirkwood, north of the Forest Road. This biome differs from the surrounding lands as it's much more mountainous and the woods are less dense, with an open canopy. The valleys and lower slopes have fewer large Mirk-oak trees. The mountains provide shelter to Orcs of both Dol Guldur and Gundabad.
The rivers in Mirkwood are like normal rivers, but they are touched by the evil taint that originated in Dol Guldur. You would do best to avoid these waters, as just touching the surface of the river gives you temporary blindness, slowness, and mining fatigue.
A convenient way of transport to cross them is either on the back of a mount, or on a Barrel. When travelling on these rivers per Barrel you get the Achievement "Out of Bond" and the title "Barrel-rider."
Only one structure spawns in the forests of Mirkwood, and its occurrence is rare. Webs of Ungoliant can be found in the trees and on the ground, making navigation tricky.
Ruined Wood-Elven Tower - On rare occasions, ruined towers could be found in Mirkwood. Once occupied by Elves of the Woodland Realm, these towers are now ruined, empty, and degraded. However, at the bottom there is a chest containing loot of many varieties. But be careful, as there is no easy way out, and many dark things dwell in the depths of these towers...
The mobs that spawn throughout Mirkwood are spiders, elk and the common animals: deer and butterfly which is a kind of purple butterflies called 'purple emperor', but pose no threat, compared to the spiders that lurk under the trees. The spiders are especially noteworthy:
Mirkwood Spider - Mirkwood spiders are plentiful. They were bred by the power in Dol Guldur, and released into the Mirkwood. They are hostile to any enemies of Dol Guldur, so beware. These spiders can spawn in different sizes, from huge to tiny. Good players should be careful, for some Spiders are poisonous, and some have venom that gives you effects of slowness. There are three types of Mirkwood Spiders: black spiders merely do damage, green spiders have poisonous fangs, and the bite of the blue spiders results in a crippling effect, slowing you down temporarily.
Next to the aforementioned, The Mirkwood Mountains host the following NPCs:
Dol Guldur Orc - Orcs of Dol Guldur bred to serve the three Nazgûl. Previously, his lord was the Necromancer, but he was cast out of Mirkwood. Now, they follow the orders of Khamûl, the Black Easterling.
Mirkwood has dark green grass. The Shadow spreading from Dol Guldur has left the forest floor flowerless.
The grass is dark, and in order to construct a fit environment, the mod introduced Mirk-oaks in Mirkwood. The forest is dominated by this kind of large, tall, broad trees with large, dark green canopies. The tree coverage is so thick, that one could cross the forest on the treetops in most places.
The undergrowth is dominated by both smaller, dark green Mirk-oaks and other trees like large oak, spruce, fir, and pine. Despite the tainted waters, willows still flourish along the banks.