I'm new to the site and this is my first contribution, so if it seems a little awkward... well, that's why! Anyway, I've been doing a lot of theorizing lately, so I thought I'd share one set of ideas. This runs contrary to current Zelda canon, but I hope you still find it interesting!
Demise's Heretic FollowersEdit
After playing Skyward Sword, I came to the conclusion that Demise shares a connection with the group shown in Twilight Princess, refered to as the Interlopers. When he is defeated as the Imprisoned, Demse behaves very similarly to the Fused Shadows, while his weapon, Ghirahim, uses magic that takes the form of diamonds, and these diamonds faintly resemble the black wisps of Twilight. Demise's "hair" (in both forms) also resembles Twilight. In TP, Midna also makes mention of a king whom "[the Twili's] ancestors lost to greed", but the name and nature of this king are not expanded on. Based on this, I believe it's possible that a group of rogue Hylians defected from Hylia's worship prior to her fight with Demise, and swore allegiance to the latter. For their loyalty, Demise gave them some of his power, allowing them to excel in magic. Ultimately, however, Demise was sealed and his followers left leaderless.
Following Demise's defeat, the tribe who would become the Interlopers went into hiding. Eventually, their aimless travels led them through the Lost Woods into the country that would later be called Termina. Here, their hatred of the goddesses grew, and they prepared to exact their revenge. To do so, they first hid away in Ikana Canyon, where they built a powerful citadel and imbibed it with incredible durability; centuries later, it would come to be called Stone Tower. Here, they forged weapons, two of which were supreme: a magical Trident (the same one seen in Four Swords Adventures) and a cursed mask (Majora's Mask). The latter seemed to be the greater of the two, and the Interlopers embossed its image on many objects all across Termina while boasting of its capabilities. Any that opposed them then experienced the mask's power firsthand, as they were hexed and tortured with its power. However, this backfired on the Interlopers, as this use of the mask caused it to be filled with darkness and negative energy. Over time, the mask became so saturated with this corruption that it developed a warped and sadistic will of its own, a will that began to manifest as tentacles pouring from the mask. Concerned, but viewing this as little more than a setback, the Interlopers placed a seal on the mask to keep its new resident entity in check. But a new threat emerged from elsewhere.
The malicious activities of the Interlopers and their new mask hardly went unnoticed, and word of their dark deeds spread far, even into the young kingdom of Hyrule. The Sheikah tribe, fearing just what this tribe and its mask could do, decided to meet the threat head-on, and a small group of them entered Termina in secret. To protect their identities, this group adopted the pseudonym of "the Ancient Ones". In time, they traced the rumors and tales of the land's misfortunes to Stone Tower, where they issued a challenge to the Interlopers. The two tribes then met in battle, and after a fierce clash, the Sheikah managed to seize the mask. Bereft of their greatest weapon, the Interlopers began to suffer ever greater casualties, forcing them to retreat. Even in retreat, however, the Interlopers benefitted, gaining some knowledge of Sheikah magic and spellweaving. They then fled back into the Lost Woods, where they resumed their experiments based on what they had learned in forging Majora's mask. In time, they managed to create a new weapon, one which synthesized the potent abilities of the mask with the versatile traits of the Sheikah: a magical helmet known as the Fused Shadow.
Meanwhile, the Sheikah returned to Hyrule, mask in hand. It soon became apparent, however, that the strictures the Interlopers had placed on the mask had withered, with the demonic entity within growing increasingly powerful. Lacking the means to exorcise the mask, the Sheikah instead chose to place it beyond the reach of people, placing it in the depths of a temple they controlled and firmly sealing it within the deepest level. To strengthen the seal, the Sheikah also sealed away a mask of their own, a mask created to honor their long-desceased goddess, Hylia, and made in the likeness of her chosen hero (the Link of Skyward Sword), known as the Fierce Deity's mask. However, centuries later, acts of torture and hatred occured in the temple, and the resulting flow of negative energy strengthened Majora's mask, allowing it to consume the Fierce Deity's mask and fracture its seal. Eventually, a man known as the Happy Mask Salesman, following stories of the mask, entered and navigated the temple (by that time known as the Shadow Temple) in secret and seized the mask. Shortly thereafter, feeling a sense of foreboding all the while, the man left for Termina.
In time, the Interlopers waged war on Hyrule, but were ultimately defeated by the Light Spirits and banished to the Twilight Realm. Their Trident was sealed deep within the Pyramid of Power, while the Fused Shadow was broken and scattered.