The Citadel Forest, now called the Sylvaneth Wyldwood, is a Citadel Miniatures model made for Warhammer Fantasy. It consists of three trees on one base. Typically used to represent Athel Loren and similar grimdark Elf forests, as well as the Empire forests like the Drakwald.
Over the years it has been derided as one of the worst value, biggest money-grabs in the tabletop industry (outside of Warhammer 40,000 products that is). This is primarily due to the fact that the buyer is spending $88 on three plastic trees on a plastic base, something that could be accomplished MUCH cheaper (and oftentimes much better) by buying aquarium trees, railroad scenery, or even just spending one hour with some papier-mâché. Regardless, Games Workshop managed to enforce purchasing it with extensive use in 8th Edition Warhammer Fantasy and Age of Sigmar by requiring it for an entire army, and in the latter for one of the primary gameplay modes. Note they are required in the plural...
The base is an oval shape, with raised rock edges poking out of the grass and several skulls (because its Warhammer scenery after all). The trees themselves are oak with ropes around the base and branches of one. Also skulls. The leaves consist of solid chunks, giving the impression of a dying tree (or metal shrapnel stuck in branches). Due to the odd appearance of these leaves, many players completely leave them off.
The trees fit into rounded bases to be removed for models to be placed on the terrain.
They were squatted later in Age Of Sigmar, and replaced with a cheaper alternative called the Awakened Wyldwood.
One of the wargaming staples, but one that costs so much nobody could be bothered to buy the damn thing. It didn't have any special rules, it was just difficult terrain. The Wood Elves get a free one. Brilliant.
Age of Sigmar
Like the Realm of Battle, it needs basing material to look like anything but shit.