Marvel's Multiverse is a collection of interrelated universes, a set of different expressions of all time and space. Since supervillains are constantly looking for any advantage they can get, it's not surprising that dark gods and demons were some of the first characters to explore the multiverse.
Among the first alternate universes introduced in Marvel Comics was the Negative Zone, a reality so old it was contracting rather than expanding. Other, more arcane, dimensional realms like Chthon's Flickering Realm were introduced later, to be explored by sorcerers and wizards. Marvel has a vast multiverse, and villains have been exploring it since its inception.
12 Loki Has Traveled Through the Multiverse for at Least 1,000 Years
Born out of Norse mythology mostly written in the 13th century, Loki Laufeyson is the half-brother of Thor. In his first appearance in Stan Lee, Larry Lieber, and Jack Kirby's Journey Into Mystery #85, Loki escaped imprisonment on Asgard and traveled to Earth. It wasn't his first trip to to the plane called Midgard by Asgardians, and he became a frequent flyer for the next several decades as he frequently battled Earth's Mightiest Heroes, the Avengers.
Most famously, in Avengers #1, Loki menaced the heroes in a plot that inadvertently led to the fomation of the premier superhero team. However, a conservative guess as to how long he has been traveling across the multiverse points to more than 1,000 years.
11 Annihilation Traveled to Earth-616 Roughly 3,000 Years Ago
Annihilation is best known as the god of Amenth, "a fallen world." A newer addition to Marvel's mythology, not much is known about the self-proclaimed god or its home realm. Annihilation's single defining feature is a mask that carries its consciousness.
Annihilation has no body, no gender, and no physical presence of its own outside of the mask, which was sent to Earth-616 and played a part in a war with the X-Men's old foe Apocalypse in Ancient Egypt. This occured about 3,000 years ago, making Annihilation one of the oldest travelers of the multiverse.
10 Ancient Peru Banished Tiboro To The Sixth Dimension
Once the ruler of a massive civilization headquartered in ancient Peru, Tiboro was magically banished to the Sixth Dimension. The original time of his reign cannot be pinpointed but must be some time after the first humans settled in Peru around 14,000 years ago. Thereafter, he became an explorer of the multiverse.
Tirboro survived in the Sixth Dimension, amassing more and more power and knowledge of the other universes. He's since emerged as an enemy of the Multiverse-traveling hero America Chavez. When he explores new dimensions, conquest is at the forefront of his mind.
9 Mephisto Travels Realities as He Collects Souls
Mephisto is not Marvel's Satan but that's who he pretends to be. He claimed he was created by the First Firmament, before the first multiverse. This would make Mephisto older than almost anything conceivable but since he's a malevolent trickster, it's hard to believe anything he says.
Mephisto visited the 616 universe from the dimension he calls Hell as early as one million BC, when he appeared in the form of a snake and created the first Ghost Rider. Devil or not, he's an ancient being whose been hunting the multiverse in search of souls almost as long as life has existed.
8 Surtur Is Norse Mythology's Lord Of Muspelheim
When Odin moved to quell a rebellion, Surtur the Lord of Muspelheim fled and was trapped in the Earth's core. His battle with Odin formed the Earth's moon and prompted the All-Father to establish the Rainbow Bridge that Asgardians use to travel to other dimensions.
Surtur's journey to Midgard happened very early in Marvel's chronology. Gods, titans, and the like are some of Marvel's most ancient beings so it's not surprising that the King of the Fire Giants is numbered among the Multiverse's earliest explorers.
7 Set Was Earth's First Interdimensional Fugitive
Pulled from Egyptian mythology, Marvel's Set was one of the first (and worst) Elder Gods. In a reflection of the Biblical Cain and Abel, Set murdered Hyppus, becoming Earth's first murderer. Originally a giant snake, Set later took many forms.
After the Elder Gods turned on one another for power, Set took a page from Chthon's book and created a new dimension to effect an escape. He expanded and explored the Multiverse by creating this new reality, the Serpent's Sea.
6 Chthon Created The Flickering Realm Before Life Appeared on Earth
In the running as one of the oldest gods of Marvel's Earth, Chthon was born from the pure life-energy of Earth years before any life formed in the planet's oceans. Mark Gruenwald and Alan Zelenetz explained all this with art by Bob Hall in the prologue to Thor Annual #10.
When the Elder Gods of Earth turned on each other and Chthon's defeat loomed, he created an entire dimension of his own called K'lay, which humans would later call the Flickering Realm. Before escaping into his universe he inscribed instructions on how to communicate with him on magically indestructible flesh, creating the Darkhold.
5 The Former Scientist Galactus Was One of the First Multiversal Travelers
Galactus, the Devourer of Worlds, was once Galan, a scientist on Planet Taa. He evaded the collapse of the sixth multiverse and was reborn as Galactus in its next iteration. Despite being created by the legendary team of Stan Lee and Jack Kirby, his origin was later written by Mark Gruenwald and were added to a reprinted Thor story.
This made Galactus one of the first beings to traverse the multiverse. Much later, Galactus was sent to alternate dimensions by heroes trying to rid the 616 Universe of his dangerous hunger. So far, the first citizen of the multiverse has always returned home, and has even had his story greatly expanded upon in 2021's Defenders Beyond (by Al Ewing and Javier Rodriguez) thanks to timeline-breaking interactions with the eponymous team and the Silver Surfer.
4 The Black Winter Has Destroyed Many Multiverses
Introduced relatively recently, the Black Winter is a malevolent force of nature. It's a pure elemental entropy that has existed since before nearly everything, and was responsible for the destruction of all existence prior to the formation of the multiverse.
The Black Winter was first seen in Thor, when it destroyed Galactus' homeworld as the Creeping Plague. Recently, the entity appeared fully formed in subsequent issues written by Donny Cates and drawn by Nic Klein. In this story, the Black Winter revealed to Thor that it caused the destruction of the sixth iteration of all reality, and continues traveling the multiverse. This means the Creeping Plague was but one facet of the Black Winter, and began exploring the Multiverse before the Big Bang.
3 The Celestials Created The First Multiverse To Spur Evolution
Before any sentient life or physical matter existed, there was the First Firmament. This was the brainchild of British-born writers Grant Morrison and Al Ewing, and is the force responsible for creating the Celestials, god-like multiversal manipulators created by Jack Kirby and debuting in 1976's The Eternals.
The Celestials nearly destroyed the simplicity of the First Firmament not so much by exploring the Multiverse but by creating it to push life to evolve and grow. Nearly invulnerable, immortal, and omnipotent, each one of these beings are over 2,000 feet tall and are among the most powerful beings in the universe. They also have the ability to travel effortlessly through the multiverse.
2 Dormammu Has Terrorized Multiple Dimensions for Countless Years
Introduced in 1964's "The Domain of the Dread Dormammu!" by Stan Lee and Dick Ayers (from Strange Tales #126), the lord of the Dark Dimension has spread his influence across realities for longer than anyone could possibly guess. As a Faltine (beings of energy born from pure magic), Dormammu is capable of unfathomable cruelty and untold destruction, and he has never shied away from letting that be known wherever he sest foot.
Although there is no clear indication as to how long ago Dormammu first explored the multiverse, it is well-known that the Ancient One spread his own influence across the Multiverse in a bid to stave off the villain in the 17th century. As such, it is hard to imagine that Dormammu isn't among the earliest Marvel villains to understand the infinite potential of the Multiverse, even if he has largely stuck to the primary Marvel Universe when it comes to his malevolent machinations.
1 The Beyonders Were Created to Traverse and Observe the Multiverse
While one Beyonder stands out from the rest, the race of beings are among the oldest and most powerful figures in the Cosmos. Originally, the Beyonders were created by the Celestials to help them maintain the order of the multiverse, and they have been able to observe almost every single facet of existence within their preview.
Unfortunately for the heroes of the multiverse, this has also afforded the Beyonders numerous opportunities to pose a serious threat to existence. In fact, it was the Beyonders who decided to destroy the Seventh Cosmos' iteration of the multiverse in its entirety, leading to the creation of the Molecule Man as a weaponized constant across all realities. Although the Beyonders weren't exactly successful in carrying out their plot, they did succeed in destroying the multiverse, clearing the way for the Eighth Cosmos and current Marvel Multiverse to come into existence.