The Power Cosmic is a limitless energy that can allow the user to do many things, such as resurrect the dead, manipulate the user's size and mass, teleportation, telepathy, creation of force fields, time-travel, superhuman strength, immortality, and much more. The Power Cosmic can be used in a powerful blast or in small doses, such as changing the color of a plant. The Power Cosmic can also be infused into the human soul, giving them a portion of the Powers of the Universe.
The greatest and most powerful wielder (and perhaps source) of the Power Cosmic would be Galactus, the Devourer of Worlds. He is rumored to be able to teleport entire galaxies throughout time and space, re-shape worlds that have been destroyed back to their original form in every detail, and even create life. Galactus often bestows a small fraction of the Power Cosmic to his Heralds, most notably the Silver Surfer, who has used it to strip mutants of their abilities (either temporarily or permanently), and more. Galactus, however, can bestow or take away the Power Cosmic at will.
With the Power Cosmic comes a Cosmic Sense. It allows anyone imbued with the Power Cosmic to be one with the processes of the universe. The Silver Surfer once described it as being able to "hear the cosmic static of the universe's electromagnetic fields and detect the pain of dying stars or the rhapsody of planets aborning".
The Power Cosmic originates from the Astral Plane.
The Old Power
The Old Power is an attempt by the Shadow Priests to recreate the Power Cosmic. This was only partially successful, as the Old Power is unstable.
Wielders of the Power Cosmic
Galactus is the bestower of the Power Cosmic and seems to be the only living source of said energy. He can manipulate every organism. His hunger is insatiable, and the only way to sate his hunger is to consume the core of living planets, thus devouring the chosen world.
Given by Galactus
Galacta: Galacta is Galactus' biological daughter and, like both her Father and Tyrant, she is one of the most powerful wielders of The Power Cosmic in existence. But unlike Galactus and Tyrant, she never uses her powers to harm anything other than alien infestations both microscopic and macroscopic in origin. The reason she does this is to keep her adopted home world, Earth, and it's inhabitants safe.
Tyrant: Tyrant has been called Galactus' "son" and was bestowed a much greater amount of the Power Cosmic than any other herald. After a conflict with Galactus, he was reduced in power but still has shown to be stronger than any other herald.
Silver Surfer: Norrin Radd is the best known Herald of Galactus. The Power Cosmic has granted him such abilities as superhuman strength, immortality, cosmic awareness, energy and molecular manipulation as well as the ability to fly faster than the speed of light.
Air-Walker: The original Air-Walker, Gabriel Lan, was much favored by Galactus. The android that replaced him after he gave his life for Galactus was not favored as much because he did not share the same passion as the original. The original Air-Walker was given superhuman strength and durability, as well as an immunity to the vacuum of space by the Power Cosmic.
Firelord: Pyreus Kril was yet another Herald of Galactus and also friend of Air-Walker. The Power Cosmic bestowed upon him the power of flight, superhuman strength and the ability to channel cosmic flame.
Terrax: The Herald known as Terrax the Tamer was gifted with superhuman strength, stamina, durability and reflexes, superluminal flight, blast power and the ability to manipulate earth.
Nova: Frankie Raye of Earth volunteered to serve a weakened Galactus and was only given a minimal amount of the Power Cosmic due to his depleted state.
Morg: Morg the Executioner was endowed with the Power Cosmic, and from this was given superhuman strength and the ability to channel energy through his axe.
Red Shift: Red Shift also received the Power Cosmic from Galactus. He was made to be superhumanly strong and durable, immune to the vacuum of space, has the ability to open dimensional rifts and generate swords supplied with cosmic power.
Human Torch: When Johnny Storm was temporarily imbued with the Power Cosmic he gained the ability to understand absolutely anything he analyzed.
Alpha Ray: A biomechanical being Galactus infused with the Power Cosmic.
Plasma: Herald of Galactus in the Heroes Reborn world.
Stardust: Lambda-Zero was given the Power Cosmic by Galactus resulting in the power to manipulate cosmic energy in addition to its already formidable natural powers.
Praeter: Christian Pastor from Oklahoma, called Pastor Mike, and newest herald of Galactus. So far, he has shown the powers of Super Speed, Flight, and Tracking.
Superman: At one point, Galactus made Superman his Herald, removing his cape and sheathing him in gold. With the Power Cosmic Superman's natural powers were enhanced by a great degree, including space travel. A host of new powers were also granted to him including firing powerful energy blasts, teleportation, unleashing a blast from his whole body - which was used to blast Mr. Fantastic off of him, cosmic awareness, and probably a whole host of other abilities which were never revealed due to him betraying Galactus. However, when Superman did have the Power Cosmic, Galactus said that he was his most powerful Herald ever. (see: Superman/Fantastic Four #1)
Cosmic Hulk: This version of the Hulk is a robot created by the Mad Thinker. Once, instead of having heralds, Galactus had a data loop program that wandered the universe, compelling beings to create artificial life forms that would one day help Galactus consume their planet.
Given by Silver Surfer
Captain Universe: The Silver Surfer permanently infused a portion of The Power Cosmic into the Uni-Power and repaired both its severe memory loss and power fluctuations.
Shalla Bal: She was granted a portion of the Silver Surfer's power. She has the ablity to grow life wherever she steps. She can also call out to the Surfer for assistance through her thoughts.
Silver Skyriders: In another universe, twelve pieces of the Silver Surfer bestowed his power on twelve aliens forming the Silver Skyriders.
Skaar: Silver Surfer wished to show Skaar what it was like when a mortal reaches for the power of the Gods, so he shows him the Power Cosmic by fusing with him. The Silver Surfer then talks to Skaar about decisions that would change his life forever, but Skaar has a different point of view.
Earl Weygand: The Silver Surfer gave half of his Power Cosmic to Earl in order to help him defeat the Enslavers. This turned Earl into a Silver Surfer himself. He was able to destroy the power supply of the ship, but it cost him his life.
Taken from a wielder
Doctor Doom: Victor Von Doom did for a time steal the Power Cosmic from the Silver Surfer. He eventually had the power stripped from him by Galactus.
Doombot: A robotic creation of Doctor Doom sent to absorb the Power Cosmic for himself once again. It was able to get the power but was destroyed.
Red Hulk: In battle with the Silver Surfer, the Red Hulk strangles him and before the Surfer could stop him, the Red Hulk absorbed every inch of the Power Cosmic into his body and kills the Silver Surfer. He then takes Terrax's axe and the Silver Surfers board but Galactus once again defeats Rulk. Grandmaster resurrects all of those killed by Rulk, including the Silver Surfer.
Cosmic Messiah: The Cosmic Messiah was created by using a combination of the Puppet Master's radioactive clay and the Power Cosmic siphoned from the Silver Surfer by the Gangers.
High Evolutionary: Ambushing the Silver Surfer on Earth, the High Evolutionary completely siphoned the Power Cosmic from the Silver Surfer, leaving him a mortal being again. Wyndam did not absorb all of the Power Cosmic into himself, but instead, used it as needed and took control of one of Galactus' Star Spheres.
Horacio de la Fuente - A dying man given the Power Cosmic by the High Evolutionary.
Suzi Endo: Suzi (also known as Cybermancer) was made Herald to the High Evolutionary and changed with the Power Cosmic, much like the Silver Surfer did. She would come to be known as the Seeker.
In the Heroes Reborn world, the cosmic rays that gave the Fantastic Four their powers were actually part of the Power Cosmic. The Heralds of Galactus were able to sense this in all four members.