After you have completed the story mode, head to Rivendell and locate the secret room behind a waterfall where you found a mithril brick before. Here you will enter the bonus level: Lego Middle-Earth.
This is a 'Legolized' representation of Middle-Earth where your goal is to wreak total destruction using Sauron and collect 1,000,000 studs. Sounds wonderful!
It's actually pretty easy compared to what you may expect. Just make sure to smash everything and don't miss out these few little details:
- If you switch to other characters while using Sauron, you cannot use Sauron again until you complete the level and get Sauron as a reward.
- Make sure you destroy every stuff in Moria, including all the Lego walls in all the chambers, and go inside the crank at The Argonath and destroy everything inside.
- Use any character with explosives to blow up the fortification at Orthanc, then destroy the towers.
- Ride the oliphaunts to acquire bonus studs.
- Use Elves at Lothlorien to jump on every possible tightropes, higher areas, and tree houses.
- Totally smash Mount Doom. Make sure to re-create the parts from it's debris on the first attack and destroy it again later.
- Smash anything, be liberal about this. As what is stated above, whenever you can build items, build them as well, you need every little bit of allowed interaction in the environment to be done.
Once completed, you will receive Sauron and The Mouth of Sauron. Congratulations!