The Otherworld: Lydia Through The Looking-glass
So a little while ago Kara wrote a theory post on the use of mirrors and reflections because they kept cropping up. Since then we’ve noticed many more uses of mirrors – to the extent that we don’t actually understand how we missed them all and are now totally convinced it’s not by accident.
To get straight to the point I think that the mirrors (and possibly other reflective surfaces) are gateways or windows to The Otherworld.
What is The Otherworld?
The religious or metaphysical concept of otherworlds such as a realm of supernatural beings and a realm of the dead can be found in many cultures throughout the world. 
And specifically in Celtic Mythology:
The Celtic Otherworld is a concept in Celtic Mythology, referring to an Otherworld such as a realm of the dead and a home of the deities or spirits. It is often described as being right alongside the world of the living, but invisible to most humans.
The Otherworld can essentially be thought of as a mirror of our world separated by a Veil. There can be more than one Otherworld, situated in more than one place or everywhere at once it’s a paradox, physics doesn’t really apply here. Normal rules also don’t apply in The Otherworld; time moves differently and people don’t age. .
Spirits are thought to travel between worlds, or layers of existence, usually along an axis such as a giant tree, a tent pole, a river, a rope or mountains  . In the first teaser for season 3. We had a rope and also a tree (you might not be able to see it but when Scott is sinking at the end there are its branches behind him).
Then in the new opening credits there are trees when Scott rises in the mist and also a tree at the end.
Also trees are generally important to mythology and also within Teen Wolf. Mountain ash is made from the Rowan tree in the trailer we see a body tied to a tree and one of the episode teasers is Oak and the Oak tree was the most sacred of trees to the Celts. So there is that.
I dunno about the tent pole though.
The only tent pole I can think about is the one in Derek’s trousers.
In some Irish stories some people or things are able to see or cross over into The Otherworld because of how it exists right alongside us but is invisible. I believe that Lydia is one of these people; she can commune with The Otherworld or they can use her as a conduit and commune through her. Interestingly Banshees are seen as an omen of death and also a messenger from The Otherworld and a lot of fandom thinks that Lydia might be a Banshee.
So what if all Lydia’s hallucinations were not hallucinations per se she is just privy to The Otherworld and Peter had a stronger connection than anything else that might be in The Otherworld because he bit her and thus was able to communicate with her through that bond. Peter also had greater influence in her dreams which could be like a place between worlds. He was using her as a tether back to the real world.
Also we still don’t know what Lydia did for those two days in the woods while she was in a fugue state. Trip to The Otherworld??
Additionally, Lydia is quite involved with mirrors often having scenes looking into them and specifically we have her breaking one in 2×04 ‘Abomination’ and significant mirror action in 2×07 ‘Restraint’ with young Peter/Peter in the wardrobe.
Not to mention 2×05 ‘Venomous’ when Lydia had her hallucination in class.
Peter blew some white powder (jury is still out on what this actually is) in her face and she wrote SOMEONE HELP ME in mirror writing on the chalkboard.
Was it Lydia asking for help or someone using her as a conduit from The Otherworld to ask for help through her? Could it be Laura Hale? Remember the girl screaming with no sound in Lydia’s hallucination on the lacrosse pitch in her dream in 2×09 ‘Party Guessed’? We heard whispering and voices quite a few times in Season 2 and now in the teaser for Season 3. It was believed in the past that the only thing that could travel across the ether between worlds was sound…curious yep.
So basically we think that the mirrors are gateways to The Otherworld if at least a way to see or communicate with it.
Now that the titles for Season 3 have been release we might be rolling on the floor. Everything seems to be hinting at The Otherworld.
Lydia is split in two via a freaking mirror effect and screaming like a Banshee.
Also there are hands reaching out through black goo, dirt, black mist?? And one reaching back!!! Trying to maybe pull someone back? Who?
Is this the veil between worlds it is described as a “fluid, diaphanous thing which can be thinned or even lifted entirely, at different places and times.” A mist or a lake can be places where the veil is thin enough to allow passage between the two worlds .
This gif possibly hints at a misty space between the layers of existence? I have always wondered in 2×10 ‘Fury’ when Derek was like this:
Where was he?
Celtic shamans journeyed to the Middle World to access The Otherworld maybe that is what it is a place between the worlds?
It is my belief that one of the ways to move between worlds (or transport) is through supernatural mist (read more about mist in Teen Wolf here) or by dying.
This might explain why Scott seems to be dead at one point in the trailer. Maybe this is the only way he could get to The Otherworld thus the symbolic nature of rising through the mist and the tree. The Celts instead of believing in reincarnation believed that you were reborn…to actually live again exactly as Peter did .
There has also been mention of a Halloween episode and in Celtic mythology Samhain is when the veil between worlds is thin and things can pass across. If this ends up being 3×06 which Jeff has said will be a homage to The Shining then it makes sense.
We know that the episode will be partially set in a Motel and in The Shining the Hotel was haunted by ghosts. So spirits from The Otherside maybe? It’s going to be really creepy. Also the teaser word for 3×06 ‘Motel California’ is Whistle. Which makes sense if you believe that Derek was called back from the white place via the whistle?