In Loki episode 5, Sylvie finds herself running from an all-consuming sentient cloud of wrath called Alioth, and uses her enchantment powers to get a glimpse of what Alioth is guarding. Her brief vision reveals a castle on a rock floating in space, and teases that someone is hidden away inside its tower.
"Journey Into Mystery" takes Sylvie and Loki one step closer to learning the truth about who really created the Time Variance Authority, since the previous episode revealed that the Time-Keepers are simply automatons. After being "pruned" by Ravonna Renslayer, Loki finds himself in what is effectively a giant landfill at the very end of time, where everything that the TVA is pruned gets sent so that it can no longer influence the sacred timeline. Among the denizens who have avoided being eaten by Alioth are several Loki variants, who now consider this place at the end of time to be their home.
Alioth instinctively targets any new arrivals in his domain, and after being cornered by Ravonna Renslayer and the TVA, Sylvie chooses to "self-prune" so that she can Loki in his desolate new location. She's attacked by Alioth immediately upon her arrival, and while running from the creature she uses her enchantment on it. This briefly offers her a vision of what Alioth is connected to: a roughly ring-shaped rock floating in space, with a castle on top of it. Her vision moves closer to the castle, showing a great pair of doors at the bottom and, at the top, a tower with a glowing yellow dome. It appears that the true creator of the TV and the Time-Keepers is hiding here, in the void at the end of time, with the fearsome Alioth as their guard dog.
The most popular theory (fueled by certain casting rumors) regarding the identity of the "man behind the curtain" is that it's Nathaniel Richards a.k.a. Kang the Conqueror, a time-traveler from the comics who journeyed forward in time and conquered a weakened and war-torn Earth, expanding his rule across the galaxy from there. Having conquered the future, Kang then reached back in time to try and conquer an earlier version of Earth. Kang is strongly linked with both the comic book version of Ravonna and with Alioth, so the chances of this castle on a floating space rock being Kang's citadel seem very high.
There's another key reference to Kang at the very start of Loki episode 5, which shows in closer detail the ruined version of Avengers Tower that has been sent to the end of time, and reveals that the tower now says "Qeng" at the top. In Marvel Comics, Qeng Enterprises was the company owned by "Mr. Gryphon" - an alias used by Kang the Conqueror when he was posing as a businessman and CEO in the 21st century.
At the end of "Journey Into Mystery," Sylvie and Loki succeed in ing their powers and are able to enchant Alioth, parting a green curtai of magic to reveal the castle that Sylvie saw in her vision. The episode ends with the two of them walking towards the castle, so it seems that the full truth about the TVA and the person who created it will be revealed in the finale next week.