It seems like a perfectly normal oak forest.
It is not. The reasons for that have been lost to a confusing morrass of history, legend, and speculation. What is known is the Dark Forest is not fit for a good man or beast to pass through, let alone live.
The Dark Oak Forest has bubbling HellPits, springs of green ectoplasmic goo. These pools of Evil change the things around the forest, making them monsters (or monsterous). Sometimes things of Evil emerge from them whole cloth.
The spirits of those lost to the forest continue to haunt it. Some are simply sorrowful, others want revenge on the living for their sorrys state.
Only once did an army march through the forest. It was met upon the other side and battle was joined. The death summoned up such evil, that both sides banded together to save themselves, in The Battle of Living against the Dead. Though advantagous, no one would dare march an army through the forest again.
Things are not contained by it. Every now and again things emerge from the forest to terrorize those around it.