- "Among those living in the light, interlopers who excelled at magic appeared. Wielding powerful sorcery, they tried to establish dominion over the Sacred Realm. It was then that the goddesses ordered us three light spirits to intervene. We sealed away the great magic those individuals had mastered."
- — Lanayru
The Interlopers are characters mentioned in The Legend of Zelda: Twilight Princess. A tribe of sorcerers, the Interlopers were most likely among the combatants of the Hyrulean Civil War, in which many races fought amongst themselves to establish dominion over the Sacred Realm and the Triforce. The Light Spirit Lanayru tells Link about these beings and their history after he restores the Vessel of Light. In Lanayru's telling, the Interlopers resemble evil copies of Link, though this is likely not their true form.
Using the dark power of the Fused Shadow, the Interlopers tried to establish dominion over the Sacred Realm, but were stopped. As a punishment, the Golden Goddesses sealed them away into the Twilight Realm, and split the Fused Shadows, the source of their evil magic, into four separate shards. At the same time, the other races that fought in the Hyrulean Civil War settled the dispute in a seemingly peaceful manner. The Interlopers would eventually evolve into the Twili.
Some fans go as far to say that the Sheikah are the Interlopers from whom the Twili are descended; being so close to the Royal Family of Hyrule, they would know much of the Triforce. Both the Interlopers and the Sheikah have red eyes and both are dark magic practitioners. Another clue to this is the Sheikah symbol carved onto the back of the Fused Shadows. However, the presence of Impaz and the fact that the Sheikah protected the Hylian royalty would seem to contradict this and thus make it improbable for the two to be the one and same, but the term "interlopers" may suggest that a group of Sheikah interfered with the loyal Sheikah, whose duty it was to protect the royal family, and wiped them out to get to the Triforce.
However, some theorize that the Sheikah and the Twili are the same race, but had a feud. One side could have wished to protect the Triforce, while the other side wanted to steal and harvest its power. This feud would have put the original race into two different sides, Sheikah and Twili. Twili also could have used their dark magic for more selfish or evil tasks than Sheikah.
In the Ocarina of Time manga, it is told that the Sheikah symbol was altered with a tear because the tribe was betrayed by the Royal family. It is possible the Sheikah, with their knowledge of the Sacred Realm, in fact actually betrayed the Royal family and triggered the Hyrulean Civil war by attempting to reach the Triforce. The Interlopers might have been rebellious Sheikah. This would explain why in the Ocarina of Time, the tribe is said to be nearly extinct : the traitors were killed or banished into the Twilight Realm, and only the loyal ones survived, with an altered eye with a tear as a symbol. It is interesting to notice that Agahnim has an eye symbol similar to the Sheikah's. This hypothesis makes the Twili actual Sheikah descendants.
Another race called the Ancient Ones, though not featured in the series and is only briefly mentioned in The Legend of Zelda: Majora's Mask, are those that used Majora's Mask in their 'hexing rituals'. There are many similarities between the Twili and Majora's Mask itself, which implies that the Twili could be the same race. The similarities involve some shapes featured on Majora's Mask being present on Midna's helmet, the Fused Shadow (the eye especially).
Zant's behavior is very similar to creatures specific to Majora's Mask and the Mask itself, such as Zant screeching in a similar fashion to Majora's Mask. During the mention of those who used the Mask, the Happy Mask Salesman mentions that the mask was hidden in shadows; the Twilight Realm is sometimes referred to as "The Realm of Shadows."
A possibility is that the Ancient Ones are the Terminan counterparts of the Interlopers. Terminan and Hyrulean counterparts are known to have differing moral views, which explains why the Interlopers attempted to use the Fused Shadow for acts of destruction, while the Ancient Ones willingly sealed away Majora's Mask themselves upon learning of its immense power.
Another possibility is that at least a portion of the Gerudo tribe were the ones who gained the mysterious power of the Fused Shadows and were cast into the Twilight Realm. This would be supported by Midna's story that the Twili race were originally a tribe of thieves, and why the entrance to the Twilight Realm is located in the Gerudo Desert. The Gerudo are no longer present in Twilight Princess, and just before Zant is fought within the Palace of Twilight, Midna mentions that the Twili's ancestors "lost their king to greed"; this may mean Ganondorf, whose desire to possess the Triforce eventually led to his defeat, though as that occurred long after the Interlopers were sealed away, she is likely referring to someone else.
As for appearance, Midna does have the classic red-orange hair of the Gerudo and her true form is tall and slender, just as the Gerudo women once were. Also, there is a Gerudo pattern and converted Gerudo symbol on the front of Zant's robe, which he wore even before his encounter with Ganondorf, whom he worshiped as a god. Gerudo also worship Ganondorf as a god, which a Gossip Stone tells Link. However, this theory seems to disregard the fact that a man is born into the Gerudo tribe only every 100 years, although it is possible that when the Gerudo evolved into Twili, this aspect of the Gerudo people was discarded. The Gerudo, if this theory is true, would likely not be the only race that the Interlopers were composed of, as there would be no males for them to mate with in the Twilight Realm were they sealed off alone, and the race would die out.
One interesting theory is that the Interlopers may have been a tribe/race related both the Sheikah and the Gerudo. It should be noted that all the known true Sheikah (as Sheik is an alter-ego taken by Princess Zelda) such as the incarnations of Impa and Impaz have been female (male Sheikah do exist in the Ocarina of Time manga but it is generally considered non-canon). This could mean that the Sheikah are a predominately female race like the Gerudo, which may explain why the number of Sheikah encountered in The Legend of Zelda series is relatively low (as usually only a single member of the tribe appears) possibly due to a combination of low birth rates and members dying in service to Hylia in Skyward Sword and the Royal Family of Hyrule. Another possibility is that while male Sheikah are rare, they may not be as rare as male Gerudo. It is possible that the Interlopers descended from the Sheikah (as their race predates the founding of Hyrule), possibly either an early tribe of Gerudo or the race that proceeded them. Another possibility is that the Interlopers' tribe were the offspring of Sheikah and early Gerudo. Either of these theories could explain the Sheikah and Gerudo symbols, as the lost tribe may have inherited the Sheikah symbol from their Sheikah ancestors and the Gerudo symbol either originated among the tribe or was adopted from their Gerudo relatives. The Interlopers knowledge of the Triforce would have been inherited from their Sheikah relatives and their inclination towards thievery likely came from their Gerudo relatives. If true, this would indicate that the Gerudo and Sheikah may have close ties in the past, but following the banishment of the Interlopers, relations between the two tribes became strained to the point they split and remained separate, while the Interlopers evolved into the Twili, who continued to use the symbols of both the Sheikah and Gerudo.
The Twili bear some resemblance to the Zuna and even dress in a similar fashion. Due to the Mirror of Twilight's desert location, and the Zuna's desert home, they could have at least some relation. The ancestors of the Zuna were the builders of the Pyramids, and may have created the Trident, a weapon full of dark power, that lies within one of them. It is possible that the Interlopers were not only ancestors of the Twili, but of the Zuna as well.
Finally, there remains the possibility that the Interlopers were not of any one race or tribe but were a cult of people made up of several different races and cultures. Evidence to support this is Lanayru's vision where Link kills his closest friend to seek after the Triforce, only to become an Interloper himself. A possible interpretation of this part of the vision is that several people throughout Hyrule betrayed their fellow countrymen to obtain the Triforce and eventually united as the Interlopers.
Once sealed in the Twilight Realm, the long term effect of the Twilight on them could have transformed them into one single race, and may explain the large differences in appearance that some Twili have from others. This would also explain the Gerudo Emblem on Zant's robe and the Sheikah Emblem on Zant's throne and the Fused Shadow.
In Cia's Tale: The Dragon of the Caves, a member of the Goron Forces refers to Cia as an interloper and assumes she is after Wizzro's Ring that was kept under seal in the Eldin Caves. It is unclear if he is actually referring to the Interlopers spoken of in Twilight Princess (mistaking Cia for one of them) or simply calling Cia an interloper for her intrusion. It is also unknown if Wizzro has any connection with them.
One possibility is that in the non-canon universe of Hyrule Warriors, the Interlopers (or one of their members) may have created Wizzro's Ring form which is said to have been worn by countless dark magic users over the centuries (and the Interlopers are known practitioners of Dark Magic) and absorbed their evil magic which later manifested itself in the form of single monstrous spirit, Wizzro. It should be noted that the Gorons had previously been entrusted with guarding a piece of the Fused Shadow, so it would make sense why they would be entrusted with guarding Wizzro's sealed ring form if it were an evil artifact left behind by the Interlopers following their banishment.