(Bold words are important)
Darach is a Druid who uses their powers for evil things “If a Druid went down the wrong path, the wise oak was sometimes said to have become a dark oak. There’s a Gaelic word for that as well - darach.” Darach is the person who is in-charge of the sacrifices which are virgins, philosophers, warriors and healers using the the threefold death (which we already know) but I believe this person is sacrificing all these humans to resurrect the Hale family. In the opening credits there’s a scene 8 hands are coming out of the ground and one hand above reaching out for them. In season 1 Derek stated that 11 people were taken by the fire, but only 8 were accounted for. Leaving 3 members suspected to still be alive Peter, Cora, and the current unknown. I believe that hand below is reaching out for the fallen Hales, is either Peter or the unknown family member.
There’s a lot of theories that Peter, Gerard or Alan Deaton is Darach but we all know that Darach was the demon form coming out of the fire which lydia saw. I do think Dr. Deaton could possibly be working with Darach because he’s a powerful Druid, and not the evil sacrificing kind. He’s very knowledgeable about werewolves, wolfsbane, medicinal and poisonous effects of various plants and substances. Deaton said he came to Beacon Hills to live a normal life and to suppress his old ways by that I think he meant his evil days. He also use to work for the Hale family and was very close to them and even closer to Derek’s Mother. Deaton also stated that Druid meant Wise Oak . That they were close to nature to keep the balance and Celtic. Darach means “Dark Oak” (which is also in the opening credits.)
I believe that Peter Hale is also working with Darach because he’s in desperate need to bring back his family. In season 1 Peter killed Laura to take her alpha powers so he could avenge his family Death and also killed all the people that dealt with the arson fire at the hales house (kate argent, Mr. Harris and etc.) but I believe when he killed Laura he knew that his plan would be that he would resurrect her with the rest of the hale family eventually. (Ana Walczak was casted as Laura Hale for Season 3). It seems that the Hale family have some tie with Druids because Derek’s tattoo is Triskelion - An artful tattoo on Derek’s back motif/logo means, “Blessed by Druids.” I did some research on Druids and found this quote "The principal point of their doctrine is that the soul does not die and that after death it passes from one body into another." This got me thinking that the druid could maybe be someone who is dead on the show but we are seeing as a new character in human form? Ms. Blake got close to Derek Hale in a short period of time. I think she could possibly be darach in human form because in 1x09 episode the “Wolf’s Bane” I noticed Peter’s nurse name was also Jennifer coincidence? She isn’t that scared of werewolves and knew more information about them. She also seems to distract Derek at times and seems suspicious most of the time. At the end of Motel California she healed Derek which meant that she was a healer (another characteristic of Darach). She also said that she sees a therapist to make it seem like she’s normal. Mr. Harris was seen working for Darach was kidnapped and tied to a tree. He begs for his life, telling the killer that he “did what you asked. I did everything. They’re going to figure this out and they’re going to find you. You still need me.” he was then killed as a sacrifice (warrior) after finishing what he was told to do. Overall I do believe that Deucalion and his pack arrived for darach but they’re distraction for Derek and Scott so they wouldn’t noticed and also that the Hale family will be resurrected but there’s still one family member out there living is maybe involved with this situation.