It was a group of hunters who capture Derek. They were somehow related to Kate and had an experimental drug they injected Derek in order to cure him, but after they injected him they discover he is a born werewolf not bitten so the drug won't work. Even worse it would make him lose all control. I remember Stiles and company including Chris come to the rescue and they find Derek about to kill himself. I think it was Chris that convinced him not to go through it, by promising not to let him hurt anyone.
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