There are plenty of cosmic beings within the Marvel Comics universe, and while none is more powerful than the One Above All, the Living Tribunal is second in command and more likely to appear than its creator. The Living Tribunal is an omniscient humanoid being as old as the universe itself.
The Tribunal's purpose is to guard the Marvel multiverse from an imbalance of mystical forces, sometimes intervening and providing final judgement in multiversal events that may have an effect on the balance of power. When a matter is not of multiversal nature the Tribunal tends to leave final judgment to the cosmic entities of that universe. As the guardian of the multiverse, the Tribunal does not have alternates, existing simultaneously on multiple planes of reality at once.
The Tribunal is vastly powerful and appears as a humanoid figure with three faces, representing the different sides of its personality. The fully hooded face on its right side represents necessity, the half hooded face on its left side represents revenge, while the front face through which it speaks is equity. All three faces must agree before the Tribunal can intervene in a matter. As the guardian of the multiverse, the Tribunal ranks higher than all 'abstract' entities other than the 'One Above All' and acts as their leader.
The first known being to interact with the Tribunal was Doctor Strange in Stan Lee and Mare Severin's "The End of the Ancient One" in Strange Tales #157-160 when the Tribunal appeared to destroy Earth as a group of extremely powerful and evil mystics had awakened. The Tribunal would later appear to intervene in Jim Shooter and Al Milgrom's Secret Wars II #6 to stop the Beyonder from destroying death. The Tribunal was instrumental in the formation of Adam Warlock's Infinity Watch, as he appeared to Adam Warlock after he obtained the Infinity Gauntlet and ruled that Warlock alone could not wield the Infinity Gems in Jim Starlin and Angel Medina's Warlock and the Infinity Watch #1.
Despite his god-like status, the Living Tribunal was found dead on the moon by Iron Man and Uatu the Watcher in Jonathan Hickman and Mike Deodato's New Avengers #8. It was later revealed in Hickman and Dalibor Talajic's New Avengers #30 that the Tribunal was killed by a group of Beyonders, who had also killed the other abstract entities.
Despite the Living Tribunal's death, the multiverse was not left without a guardian. In Jim Starlin and Ron Lim's Thanos: The Infinity Finale #1 the Adam Warlock of Earth-19141 became the new Living Tribunal after absorbing the essence of the original that was left in his Universe, and then making a deal with the One Above All to cement the transformation. The new Tribunal has continued to operate post Secret Wars, with his first action being to rule in favor of Galactus the Lifebringer in Al Ewing and Travel Foreman's Ultimates 2 #2, stating that the cosmic hierarchy of this new universe was still being formed. Lord Chaos and Master Order took advantage of the Tribunal's ruling and killed him. He was later returned to life in Ewing and Foreman's Ultimates 2 #100 after the First Firmament, the being who influenced Chaos and Order to kill the Tribunal, was defeated. Since then the new Living Tribunal has continued to operate as normal.