In fact there were few big enemy's that wanted to crash all that have been built in Middlearth.
Great evil and treat for all beings was - Morgoth also knowed as Melkor.
Sauron, that you watched in movie was a little baby compared to his master - Morgoth, he was made of hate, evil and all he wants was to destroy everything. He was defeated and if you really want to know how read Sirmarilion by Tolkien. Its sometimes boring but its great book.
After Morgoth in fact she was living (?) in the same time as Morgoth, was Ungolianta, gigantic deamon spider, he poisoned and destroy Elven Sacred Tree and stole precious gems - Sirmarils when she eat those stones, her power grow so high that Morgoth was frightened...anyway about her is also in Sirmarilion.
On the picture you can see them both