It is mostly populated with Infested Bats and Silverfish, and Endermen will also spawn too. Rare variations could make the trees taller, have special Silverfish dungeons with double spawners and more loot, give terrain more hills or place much more Thorny Bushes on the surface.
The main effect of this area is infestation. Simply by standing on the End Stone Terrain, the infestation level will rise. After leaving the area for several minutes, if the infestation level is low, it will reset and won't do any harm to the player.
However, if the infestation level gets high enough, it will trigger one or more potion effects, which include weakness, slowness and mining fatigue, and if the player's level is severe, blindness, confusion and/or poison may take place as well. These effects cannot be cured using milk, you can however drink Infestation Remedy to shorten them.
Silverfish Dungeon Edit
Each chunk has a chance of spawning Silverfish Dungeons, which often spawn near caves, and sometimes on the surface too. Each dungeon has one or two silverfish spawners which are activated from large distance, thus making it easier to detect but also harder to battle, and a chest with several items such as paper, books, enchanted books and armor, food etc. Breaking a spawner also causes nearby Stone Bricks to turn into Silverfish.