Seraphim

Seraphim are a higher class of angel, above normal angels such as Uriel and Joshua, but lower in status than archangels. They've worked under the command of the archangels for at least 6 millennia. The only known seraphs are Zachariah and Castiel. The latter is only alive, though, due to losing his original grace and being powered by Theo's ( and more recently Adina's), meaning that he is no longer a Seraph.

Physical Appearance
Their true form is very intimidating, estimated to be thousands of feet high. They have four faces, one of which is that of a lion, a human, an eagle, and an ox, though this may be specific to Zachariah. They also have six gigantic feathered wings. Exposing his seraph form, Castiel instilled tremendous terror in the demon Crowley, causing him to flee.

Background
Zachariah was shown to be extremely condescending towards demons due to his pride and superiority, and of course the fact that they detest each other by nature. Seraph can easily smite lower demons with minimal effort, where a regular angel required far more focus to do so, however, it is unknown if they are capable of smiting the higher demons such as Knights of Hell, or white/yellow eyed demons. Castiel, even in a weakened state, was able to intimidate the King of Hell into fleeing when he appeared like he was about to try to smite him.

When God resurrected Castiel after Lucifer killed him, He chose to upgrade his celestial status considerably, making him a seraph. When Castiel regained his memories he was much more powerful than before, being able to smite several demons effortlessly. When he was a normal angel it took him a great deal of focus to kill demons with the touch, and he was not strong enough to kill demons like Alastair, but with his new seraph power, in a weakened state he was capable of killing Crowley, the King of Hell, although even as king, Crowley was not as powerful as white eyed demons, so it is unknown how Castiel, even at full power would fare against one.

In the hierarchy of Heaven, the seraphim, like Zachariah, are ranked higher than Cherubim and angels, but lower than archangels. Since Michael and Lucifer are sealed away in the cage, Castiel might be one of the most powerful and highest ranking angels alive, though Naomi appears to be above him. It is currently unclear if Naomi represented a higher rank than the seraph as she was clearly more powerful than seraph Castiel and held more authority.

Unlike standard angels, Seraphim powers are imbued within them, meaning they still possess all of their powers, even if they become fallen angels, and are cut off from Heaven, as demonstrated by Castiel when he burned a monster to death with his mind, during his time in Purgatory unlike the first time he Fell and lost that power because he was cut off from Heaven. The major difference between average angels and the Seraphim is that the latter possesses several cosmic powers lower angels don't, and some they do, but at a greatly elevated level. They can bend time without effort, heal humans of any injury or illness, revive humans who have been reduced to atoms, even years later. However, being separated from Heaven for long periods may weaken them as when Castiel returned from Purgatory, he still had all his powers, but it exhausted him to use his strongest abilities. This was demonstrated when he was temporarily affected by one of Crowley's demon telekinesis, but this only slowed the seraph and he was able to smite the demon.

Powers and Abilities
Seraphs innately possess all the standard powers and abilities of normal, low-tier angels, only at a higher level. Unlike normal Angels, they don't lose any of their powers after they fall.
 * Angelic Possession - The seraphim of Heaven require vessels to manifest physically on Earth, and, like all angels, they must bear their consent.
 * Immortality - Seraphs, like all angels, by nature, can't die by any natural means of death. They are impervious to bullets, knives, bombs, and any other conventional human ways of killing.
 * Superhuman Stamina - Seraphs never tire or get fatigued. As such, they do not require food, sleep, or oxygen.
 * Teleportation - Seraphs can teleport from one place to another instantly. One seraph, Castiel, was able to poke into Lucifer's Cage and grab Sam, but couldn't get his soul from the grasp of Lucifer and Michael.
 * SPN_09dfsfs.jpg]]Super Strength - As a higher class of angel endowed with greater strength, seraphs are extremely strong. They can lift at least 1 ton, as demonstrated by Castiel when he lifted an anvil with little difficulty, and overpower and destroy humans, spirits, monsters (as well as Alphas), and low-tier angels, but are completely inferior against the archangels. Seraphs are also strong enough to fight two Leviathans for at least a short time when other angels are easily overpowered by them. Castiel was able to rupture a stone wall just by touching it.
 * Biokinesis - Seraphs manipulate or influence a mortal's body, such as inflict pain down to the molecular level, like causing internal bleeding, removal of lungs, and hemorrhages.
 * Telepathy - Seraphs can easily read the minds of humans, and lower Angels, such as cupids.
 * Advanced Pyrokinesis - Seraphs can generate and manipulate fire.
 * Advanced Healing - Seraphs can heal others from severe wounds and diseases and even monster infections with ease. Castiel was able to regrow a cut-off finger on a human. However, some things are beyond their power to heal as when Sam Winchester was damaged on a subatomic level from his trials, Castiel said it was beyond his power to fix.
 * Resurrection - Seraphs can lift the deceased from death, as one of them brought Adam back to life, and another brought Bobby Singer back.
 * Invisibility - Seraphs can become invisible to humans.
 * Smiting - As all angels, Seraphim are able to burn out the eyes of humans, monsters, and demons, killing them.
 * Dream Walking - Same as Angels Seraphs can also enter humans dreams demonstrated by Zachariah when he appeared in Adam's dream
 * Regeneration - If their vessel is in some way injured, Seraphs can regenerate very quickly, though wounds caused by angel weapons take longer. They are immune to Earthly diseases.
 * Advanced Chronokinesis - Seraphs can travel through time, with greater ease than lower angels, and they are also able to send others either forward or backwards through time, with the same ease. They can send others through time without having to travel with them as well.
 * Reality Warping - Seraphs possess low level reality warping abilities, Castiel was able to assemble a game by simply shaking the box. Zachariah was able to undo all the damage Dean did to the Green Room, in a blink. 
 * Mental Manipulation - Seraphs can manipulate, erase or restore a person's perceptions and memories of events. Zachariah was able to block Dean and Sam's true memories and implant false ones. He was later able to restore their true memories. Castiel was able to erase Lisa and Ben's memories of Dean and later show Dean the true version of what happened when he escaped Purgatory. He was also able to transfer Sam's madness into his own mind. They can even enter a person's mind and bring others inside that mental world, but require physical contact with both people to do so.
 * Soul Reading/Channeling - One seraph has shown the ability to read a person's soul to check the marker another angel had left there, see if there is a soul there and check the state of the soul. He has also displayed the ability to channel power from a human soul, though this had to be done very gingerly and could result in the death of the human if done wrong.
 * Power Removal - Seraphs are capable of removing powers from empowered humans, however, it can leave them mentally damaged to do so and is very painful to the individual losing the power.
 * White Light (possibly) - As a seraph, Castiel was able to emit holy white light, but this might be due to the fact that he was powered by over 50,000 souls. Also, Naomi, presumably a seraph, threatened to use her holy light white against Crowley, before he fled.

Weaknesses

 * Angel blade - As demonstrated when Dean stabs and kills Zachariah.
 * Archangel Blade - Even more powerful than Angel Blades.
 * Angel Banishing Sigil - When activated, such a mark can temporarily banish Seraphim.
 * Archangels - As angelic beings found on the highest echelon of the heavenly host, archangels can easily kill seraphs. Raphael, who was much weaker than Michael, certainly enjoyed and took his time beating the seraph Castiel to a bloody pulp, effortlessly. When Castiel was so beat up he was immobilized, Raphael was about to kill him when Balthazar intervened. Zachariah believed that when Michael came to give him a second chance, he was going to kill him and didn't even try to fight back.
 * Leviathans - Being older than angels in general, leviathans are able to overpower and kill seraphim and prevent them from using their abilities such as teleportation when they want. However, Seraphs are somewhat capable of holding their own against the Old Ones and are capable of pushing them back even when fighting two at once. However, it appears they cannot do them any lasting harm giving the Leviathan a distinct advantage and leaving them vulnerable to attack.
 * Eve - Like the Leviathans, Eve was older than angels and showed the ability to prevent them from using their powers. She also suggested she was able to kill Castiel, a Seraph, when she told the Winchesters she would exchange his life and Bobby's as a bonus for the Winchesters' agreement to work for her.
 * Common Angelic Weaknesses - They are vulnerable to Holy Oil, Heaven's weapons, etc.

In Lore
In lore and Christianity, Seraphs are a type of Angel that are known for continually worshiping and praising God, which they do day and night. Their form is burning and fiery, and has six enormous feathered wings, two covering their face, and two covering their feet, and two with which they fly. Seraphs are claim to be so powerful and holy, that any human that looks upon them in their true form will die. unlike other angels in Christian lore, seraphs have the ability to purify sin, control and manipulate fire, light, and inflame human emotions and thoughts. Even inflame Gods holy love on to a human as well.

Along with the Cherubim (which have four faces - ox, lion, eagle and man - and four wings and also cow's feet) and Ophanim/Thrones (celestial wheels with inner wheels covered in eyes and sometimes wings), the Seraphim typically represent the highest sphere of Angels in Christian lore, and it should be noted that although 'Archangel' is the second to lowest rank, Satan/Lucifer has been named as a Cherub and a Chief Seraph in some texts.

Judaism, by contrast, ranks Seraphim as fifth in the angelic heirarchy.