The Twilight Forest Mod 1.13 realm is an endless world surrounded with densely forested. It has more of an enchanted or faerie tale feel than the main Minecraft world. The skies are perpetually dim, giving a darker, somewhat gloomy cast to the world below.
An overstory of larger trees further shades most of the world below, creating a canopy covering most of the world (so thick that you can actually walk on it most places.) The canopy is pierced only occasionally by massive trees that rise to the ceiling of the world. The terrain is flatter, or at least less mountainous than regular Minecraft, but you will occasionally encounter hills, sometimes rising far above the canopy level. These hills are hollow too, containing caves filled with valuable ores, treasure and dangerous monsters.