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After all that happened with the Darach and Alpha Pack, Derek leaves town without a word which kind of ticks off Stiles, but then there are water hags and goblins and visiting packs to deal with and time moves on. Stiles and Lydia grow closer, she as his anchor for the darkness within him, he just thrilled she's talking to him, until the inevitable happens and they have sex then start dating (trust Stiles to do it backwards). Everything's great and then Derek returns and Lydia reveals both a secret and something she's known for a long time. Stiles is mostly confused and Derek constipated, but she's the alpha female and will have her way.