2. The shield of Cthulhu was dropped by the eye of Cthulhu formed at night.
3. The probability of naturally forming the eye of Cthulhu is very low, and suspicious eyeballs are used for artificial summoning.
4. In expert mode, the eye of Cthulhu fell into the treasure bag after his death, picked it up and put it in his backpack.
5. Open the treasure bag in the backpack and get the Cthulhu shield.