The werejaguar is a sufficient castor of magic.
The werejaguar of myth, the Nagual-jaguar god, Tezcatlipoca was worshipped at such a location by the ancient Aztecs, La Iglesia. Though Kate Argent was a newly transformed werejaguar, she was drawn to the temple of La iglesia by instinct. It was there she discovered and found two Berserkers, which became her servants due to her status as a Nagual.
In Season 4, Kate cast a spell over Derek Hale. She took Derek to La Iglesia, deep into the catacombs. Sealing him in a tomb, Derek was placed under the magic of Tezcatlipoca.
Derek was then afterwards, regressed in age, status, body and mind to his sixteen-year-old self as per Kate's requirements. He possessed no memories of his present time self, having no knowledge of Kate's sociopathy and her murdering his family.
The nagual-spell cast upon Derek was broken, wore off when Derek threw himself into battle to protect Scott's Pack from Berserkers, shapeshifting to his werewolf shape; he returned to his natural age and mental growth. The nagual spell also served to trigger a Wolf Evolution in Derek.