Guardians of Knowledge/The Watchers/The Unnamable/Old Ones
While no definitive account exists, I have tried my best to provide an overview of available information on the above mentioned entities. These are referenced across religions and nations and historical accounts.
According to the Guardians, mankind were created in the hidden city of Syrn. (Note, this conflicts with the Dark Elves story of creation which places dark elves as the first species, though also in Syrn.) In the Guardian narrative, magic was given as a gift to these first beings. Due to the violence and selfishness, mankind was expelled out of Syrn in order to populate the world under the guidance of the Guardians. But at some point, a threat arrived that was so serious that the Guardians had to leave Mundus behind. They left to face the threat, which many have called “The Watchers.”
From my research, there is not one agreed upon theory. The Vrai Foi is said to have claimed that the Guardians are similar to beings like Tenamuach who many regard as a demon from another plane while others religions and scholars seem to mix information about the Guardians and The Watchers.
According to the account by the Hibou Argent, the Watchers came from beyond the world and could only enter Mundus through designated doors, or 9 monoliths centred on points of telluric energy. I have not been able to confirm that the Watchers and the entities referenced as the Unnamable (sometimes called the void) are the same or how they might relate to the Guardians or “The Old Ones” referenced in lore of the religion of Fel'Lökla.
Barring future research, I conclude that there are some sort of ancient entities with immense power and interest in Mundus who can enter in and out of our world via portals.