Your favourite piece of headcannon

One of the best things about 40k is that it leaves a lot of the truth of things so open to interpretation.  I've got a couple of theories that I really rather like. Anyone else got any interesting interpretations?

Here's some of my favourites (first two are my own thoughts, the others are picked up from elsewhere):

Necron Bio-transference
Note that they say that all Necron citizens were turned into warriors, not just Necron soldiers.

So the warriors you see marching up the battlefield were once men...women...children.

In my headcannon, their sould remain in their robotic bodies as a power source. The last moment they remember is blacking out from their normal day-to-day Necrontyr citizen life and waking up in an unfeeling, cold metal casket before fading away from consciousness.

The noise Necron warriors make when they're 'killed' is a sort of metallic rasping scream.

This is the soul that powers them flashing briefly into consciousness, continuing a 65 million year old scream of utter terror and confusion.

The idea that the implacable Necron warrior you've just triumphed over contains the soul of a long-dead innocent little Necrontyr child really brings home the heart-breaking tragedy of what happened to the Necrontyr.

Wraith Units
The reason the Eldar are so angsty about using souls to power their war machines is that is exactly what the Necrons do, the enemy they were very literally created to fight.

Mandrakes
Mandrakes are Dark Eldar touched by Qah, the last remaining Old One/manifested deity of the Hrud.

Cegorach and The Deceiver, and other Eldar Gods
Cegorach is a product of the racial memory of The Deceiver during the War in Heaven as he played both sides of the conflict. The other Eldar Gods are mirrors/representation of the Old One factions/races during the War in Heaven too.

Cegorach's Grand Plan
Cegorach's grand plan is to unite all the disparate Eldar factions and lead them into battle against Slaanesh...where they will be slaughtered to a man.

The death of all Eldar with 'birth' Ynnead and defeat Slaanesh.

He is a trickster god after all...

Short little men in cloaks that follow Dark Angels
The Watchers in the Dark are Squats

Oh, another one.

That abortion of a Necron fluff reboot was primarily Imperial propaganda to make them seem like petty squabbling in-laws rather than a genuinely scary threat...