Loki has given fans of the Marvel Cinematic Universe a glimpse into the innermost workings of the MCU. The titular villain-turned-antihero has seen his own powers dwarfed by that of the Time Variance Authority, leaving him almost helpless to truly escape their grasp. Governing all things time-related, the TVA uses devices known as Time Turners to ensure that their prisoners stay where they've been put. Loki is far from the most powerful being to be embarrassed in such a way, which the Marvel cosmic hero Adam Warlock knows all too well thanks to the Living Tribunal.
As shown in 1992's Adam Warlock and the Infinity Watch #1 by Jim Starlin and Angel Medina, the Infinity Gauntlet that Warlock is usually charged with guarding is far from the ultimate power in the Marvel Universe. Almost immediately after defeating Thanos, Adam Warlock found himself on trial by the same celestial beings that had once been on his side, and standing as the final judge was the Living Tribunal.
Making their first full appearance in 1967's "The Sands of Death" by Roy Thomas and Marie Severin in Strange Tales #158, the Living Tribunal is an unfathomably powerful, three-faced celestial being that answers to the One-Above-All. While presiding over Adam Warlock's case, the Living Tribunal witnessed the hero unleash his rage upon the court. No matter how powerful Adam Warlock is, the Living Tribunal is always stronger. With nothing more than a snap of its fingers, time was turned back to before Warlock's unfortunate outburst.
While the Living Tribunal is a far greater entity than the Time-Masters who created and ultimately lord over the Time Variance Authority, the power he displayed during Adam Warlock's trial is not all that dissimilar to what the Time-Masters' organization has been able to achieve. Like all of their other prisoners, Loki is outfitted with a collar that is remotely controlled by a Time Turner. Whenever he steps out of line, it only takes the push of a button to send him right back to when he was.
Not only that, but while attempting to reclaim the Tesseract that he had previously stolen, Loki discovered a veritable treasure trove of Infinity Stones being used as paperweights. All of these immensely powerful artifacts had been rendered as inert as Loki's own powers by their place outside of the universe of their creation.
In both the Marvel Cinematic Universe and the stories that preceded Warlock's trial, the impossible power of the Infinity Stones was put on full display. However, these stories immediately pulled the rug out from under them, indicating that they weren't the only powerful objects in the Marvel Universe. By explicitly showing that there were forces that were far greater than the Infinity Stones, these stories helped keep the mysteries of the Marvel Universe alive a while longer.