Prime Eternal is the title given to the leader of the Eternals

Eternals (comics)

The Eternals are a fictional race of superhumans in the Marvel Comics universe. They are described as an offshoot of the evolutionary process that created sentient life on Earth. The original instigators of this process, the alien Celestials, intended the Eternals to be the defenders of Earth which...

, a fictional race of godlike immortal

Immortality

Immortality is the ability to live forever. It is unknown whether human physical immortality is an achievable condition. Biological forms have inherent limitations which may or may not be able to be overcome through medical interventions or engineering...

s in the Marvel Comics

Marvel Comics

Marvel Worldwide, Inc., commonly referred to as Marvel Comics and formerly Marvel Publishing, Inc. and Marvel Comics Group, is an American company that publishes comic books and related media...

 Universe

Marvel Universe

The Marvel Universe is the shared fictional universe where most comic book titles and other media published by Marvel Entertainment take place, including those featuring Marvel's most familiar characters, such as Spider-Man, the Hulk, the X-Men, and the Avengers.The Marvel Universe is further...

. The Prime Eternal is official political leader of the Eternal city-state

City-state

A city-state is an independent or autonomous entity whose territory consists of a city which is not administered as a part of another local government.-Historical city-states:...

 of Olympia

Olympia (comics)

Olympia is a fictional city in the Marvel Universe. It first appeared in Eternals #5 and was created by Jack Kirby.-Fictional history:...

, but their power is also more than political, enabling them to initiate a Uni-Mind

Uni-Mind

The Uni-Mind is a fictional composite being in the Marvel Universe. The Uni-Mind and the Eternals were the creation of Jack Kirby.-Fictional character biography:...

, the fusion of many Eternals into a massively powerful collective organism. Although this ability is apparently inherent in Thena

Thena

Thena is a fictional character appearing in the Marvel Comics universe. She is a member of the Eternals.- Fictional character biography :Thena was born in the city of Olympia in ancient Greece, and is thus one of the Eternals of Olympia...

, so the matter is somewhat unclear. Past Prime Eternals have included Kronos

Kronos (comics)

Kronos is a fictional character that appears in comic books published by Marvel Comics. The character first appears in Iron Man #55 and was created by Jim Starlin.-Publication history:...

, Zuras

Zuras

Zuras is a fictional character appearing in the Marvel Comics universe. He is a member of the Eternals.-Fictional character biography:Zuras was born in Titanos, first city of the Eternals, and is the son of the Eternals Kronos and Daina, and brother of A'Lars...

 and Thena

Thena

Thena is a fictional character appearing in the Marvel Comics universe. She is a member of the Eternals.- Fictional character biography :Thena was born in the city of Olympia in ancient Greece, and is thus one of the Eternals of Olympia...

. The most recent Prime Eternal is Ikaris

Ikaris

Ikaris is a fictional character, a member of an alien race known as the Eternals in the Marvel Comics universe. Both Ikaris and the Eternals were created by Jack Kirby.-Publication history:...

. After Zuras' death the following position-holders would be enhanced to equal his level of power in all areas where he exceeded him/her. For example, Ikaris was previously somewhat stronger and swifter and would not be lessened in these areas, but turns into a more adept energy- and matter-manipulator.