Conversation so far:
What title did Brock Lesnar win in 2008?
In November 2008, Brock Lesnar defeated Randy Couture to become the UFC Heavyweight Champion.
When did Brock Lesnar lose the UFC Heavyweight Champion title?
Brock Lesnar was defeated by Cain Velasquez for the title by TKO in the first round on October 23 at the Honda Center in Anaheim, California at UFC 121.

Question: When did he retire?
Self-contained rewrite: When did Brock Lesnar retire?