Alas, a shadow lies once more over the southern forest. The broken cap of Dol Guldur towers over the dark woods, issuing noisome mists that blot out the sun and deepen the shadows. Wicked creatures stalk this darkness, driving away the forest animals and taking foolhardy travellers to their doom.

The Great Spiders

Not even the Elves are certain of the origin of the Great Spiders of Mirkwood. In fearful tones some mutter that they are distant memory of the spawn of Ungoliant, whose filthy broods in turn mated with lesser spiders to produce the wily arachnids of the forest.

The Great Spiders are huge, their bloated bodies sometimes larger than a man’s, spotted with lurid yellow, green and orange splashes. Black hairs cover their spindly legs and bodies. They stink with the rank odour of rotting meat.

As detailed in The Hobbit, the spiders are quite intelligent and live in small packs. They speak and work together to snare their victims, who are bound and left to soften for a while before the sweet juice is shared.

Many of the giant spiders were driven from where Bilbo and his companions encountered them in Northern Mirkwood during the wars of the Third Age. Though the spiders were entirely unconcerned with whether Sauron won or not, they suffered harshly by the hands of the victorious wood-elves.

Fortunately for the arachnids, the Elves have retreated to their woodland homes in the following centuries, allowing the nefarious broods of spiderlings to colonise wide reaches of the southern forest. The dour woodmen claim that some have returned to their lairs in the north while the Elves feast and make merry.

The Peak of Dol Guldur

The broken summit of Dol Guldur is a live volcano, though it has not erupted for countless centuries it still emits occasional wafts of sulphurous fumes. In the Third Age the Necromancer was drawn to the summit - perhaps because his treasure lay nearby in the bed of the Anduin. Doubtless the searing gasses and poisons of the troubled mountain suited his evil work.

Over the following years the Necromancer gathered evil creatures to his side and began the construction of a great tower over the natural caverns of the Dol Guldur. When Gandalf unmasked the Necromancer his fortress was tall, looking out over the black and grey reek that now hung over the forest, and delved deep - to pits, dungeons and breeding chambers below. It was here in the hot and fetid belly of the volcano that Sauron dwelt and performed many heinous crimes against the children of Ilúvatar.

The evil that was Sauron is gone and his towers thrown down, but the dreadful evil of his labours cannot ever be wholly undone. Galadriel threw open the pits of the mountain of Sorcery and cleansed them, but none dug beneath the broken slag of the mountainside to unearth every chasm or ever-dark chamber. Black smoke gathers over the hill, and the woodmen fear to go where Elves no longer tread, for they believe the twisted offspring of the Necromancer’s sorcery live yet beneath the earth.

It was in this way that he learned where Gandalf had been to; for he overheard the words of the wizard to Elrond. It appeared that Gandalf had been to a great council of the white wizards, masters of lore and good magic; and that they had at last driven the Necromancer from his dark hold in the south of Mirkwood.

“Ere long now,” Gandalf was saying, “The Forest will grow somewhat more wholesome. The North will be freed from that horror for many long years, I hope. Yet I wish he were banished from the world!”

“It would be well indeed,” said Elrond; “but I fear that will not come about in this age of the world, or for many after.”

- The Hobbit

But at the end of the Third Age a troll-race not before seen appeared in southern Mirkwood and in the mountain borders of Mordor. Olog-hai they were called in the Black Speech. That Sauron bred them none doubted, though from what stock was not known. Some held that they were not Trolls but giant Orcs; but the Olog-hai were in fashion of body and mind quite unlike even the largest of Orc-kind, whom they far surpassed in size and power. Trolls they were, but filled with the evil will of their master a fell race, strong, agile, fierce and cunning, but harder than stone. Unlike the older race of the Twilight they could endure the Sun, so long as the will of Sauron held sway over them. They spoke little, and the only tongue that they knew was the Black Speech of Barad-dűr.

- Appendix F, The Lord of the Rings