How do I protect myself if I lend money to an investor?

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Considering a personal loan to an individual for a real estate investment. The request is a loan with an interest of 12.5% paid annually x 2 years. After two years the initial loan amount is returned. Seems straight forward. 

The question I have is regarding if the individual defaults. What should happen if he does? What are the bare minimum requirements I should request in a written agreement/legal document with this individual to best protect my asset in case his investment fails or falls short. Thanks.