The Dead Wood. The trees are grey and leathery, with black, swollen leaves, because they are undead. There is no undergrowth, just mold and fungi (including such monster types as random encounters). Zombi Dryads feast under the black leaves, and rumors say enormous undead Treants lurk in the most remote areas of the wood.
The Evil Forest. All vegetation is evil to non-animal beings. Roll 1d6 every hour: a random character stumbles for 1d4 damage, or is stung by some poisonous vine: Save vs Poison or suffer -2 to all d20 rolls for 24 hours.
The Forest of Suicide. Centuries ago, during a period of famine and war, the poorest of the villagers came to this woods one at a time and hanged themselves, to remove a mouth to feed from their families. The pain and horror of these acts haunts the place, and those who travel in the forest can meet specters, voices and new suicides hanging from tree branches. Heroes failing a Save vs Spells begin to feel the desire to commit the same act and should they fail three consecutive days will have to be physically restrained from doing it.
“Help me... Please, help me...” This wood is infested with forest hyenas, treacherous and cruel animals digging burrows in the ground and among the roots of trees. They know how to imitate the human voice and utter a few words that they have learned from their victims (screaming, "help" "mercy" and so on...) and use this skill to lure explorers and assault them in a pack. A WIS roll allows one to understand there is something wrong.
The Dance of the Alders. Spending the night in these woods is very dangerous. Around midnight, distant sounds of flutes and dancing lights might attract explorers to a glade where dark creatures, looking like bewitched elves, are dancing in the moonlight. The creatures invite everyone to dance with them. If a person refuses three times or fails the dance (a DEX roll), then one of the creatures will strike him, causing a terrible disease (mummy’s rot, for example). Dancing with the creatures is just exhausting: non-stop dancing until dawn, but it will not bring any other result.
Fields of Asphodel. Small white flowers grow in this forest, capable of stealing the lives of explorers through their breath. Everyday travelers must pass a Save vs Death or temporarily lose 1 point of Constitution. As they lose their points, the asphodels around them become more vivid and florid.
Groans from the Trees. This forest resounds with groans that seem to come from everywhere. There are no green plants, and no straight branches: all vegetation is dark, gnarled and twisted, with no fruit but only poisoned thorns. The groans come from the trees themselves. Snapping a branch or hitting the bark with a blade releases cries, blood and words: the trees are miserable men turned into plants by the ancient Witch of the Woods who lives inside the forest.
Ironwood. This dark forest’s trees have deep red foliage all through the year. All the beasts living here have a 10% chance of being mutated with a partially metallified hide or carapace that grants +4 to their Armor Class and may be employed to create special armors.
Crawlwood. The trees of this forest live in symbiosis with long lived giant snails: most trees grow on top of the snails’ partially flattened shells. During the night each tree crawls for about d6 feet and all the forest is slowly moving towards the same direction. The undergrowth is scarce, as it is consumed by the snails and the soil is often slippery with the slime left by the snails.
Forest of Madness. Evil spirits haunt this forest: for each night spent here, one random character will hear a voice in his head ordering him to kill one of his comrades. If the character refuses to obey, he has to Save vs Spell or is forced to harm himself, continuing until restrained, dead or unconscious, or until a cleric successfully turns away the evil spirit, or until the break of dawn. Lone travelers are unaffected.