The Faceless Ones were evil gods, and the rivals of the Ancients. The Sceptre of the Ancients and the 4 other God-Killers (spear, bow, dagger and sword) were the only known weapons able to destroy them. However, it is said in the fifth book that one who knows their True Name has the power to take on a Faceless One without the weapon. In the sixth book it is also mentioned that the Death Bringer could fight a Faceless One without the Sceptre. It has been implied that the Faceless Ones are based on Howard Phillips Lovecraft's Cthulhu Mythos series, although it is said Lovecraft based his dark gods on the Faceless Ones (most likely a reverse of the actual truth, especially given the fact he passed away in 1937). During Kingdom of the Wicked when Valkyrie travelled to the alternate reality the Sceptre of the Ancients and one of the God Killers was present, meaning that in that reality there were Faceless Ones as well. Meaning that there are trillions more banished to other realities, so it was extremely lucky that they have not been invaded by alternate reality Faceless Ones then. Their true appearances have never been shown in the books, as laying eyes upon them is enough to drive you insane.
The Grotesquery was a hybrid of a Faceless One (the torso) and other magical creatures. When Valkyrie destroyed the Grotesquery, at the battle at Clearwater Hospital, Mr Bliss attempted , and failed to destroy the parts which came from the Faceless One. He did, however, succeed in burning the parts that were not made from a Faceless One. At the end of this book, all that remained of the Grotesquery was its Faceless One torso.
In the third book, the Faceless Ones were finally seen. In The Battle of Aranmore Farm, Fletcher Renn was forced to open the portal. Only three Faceless Ones escape out of the portal as Fletcher escapes when Jaron Gallow loses his grip on him. Valkyrie feels an incredibly powerful shock wave when the Faceless Ones are released through the portal. The three escaped Faceless Ones took over the bodies of Batu, Murder Rose and Gruesome Krav. Jaron Gallow was potentially one of the offered vessels for the Faceless Ones, however he diverted attention from him when he slices off his arm with the "invitation" sigil branded on. The Gruesome Krav-Faceless One was destroyed by Valkyrie using the Sceptre of the Ancients. The Murder Rose-Faceless One destroyed the Sceptre and in result destroyed itself as the crystal powering it crumbled. Lastly, the Batu-Faceless One was tricked back into the portal, but took Skulduggery Pleasant along with it.
In Dark Days, Valkyrie went into the Faceless Ones' realm. In there, the Faceless Ones took turns possessing the body of Batu to torture Skulduggery by slowly pulling him apart and leaving him to put himself back together. He also has constant nightmares where illusions become reality - one of which is Valkyrie when she comes into the reality to rescue Skulduggery. Because of this, Skulduggery does not believe that it is Valkyrie, and does not follow her to the portal. However, when they are attacked by Riders and the Faceless One commandeering Batu's body, China Sorrows comes through the portal additionally, taking Skulduggery and Valkyrie out whilst fighting numerous Riders.
In the finale of the book, while trapped inside an illusion which made her believe that she had been victorious, Darquesse opens a portal to the Faceless Ones' dimension and heads inside to kill them. However, the illusion made her believe she was stronger than she actually was, so when she entered the dimension it is implied she only had a very slim chance of surviving against the Faceless Ones. Skulduggery Pleasant states that, while Darquesse could perhaps kill a Faceless One, or maybe even two, she wouldn't be capable of defeating an entire race. This, incidentally, is how Valkyrie, Skulduggery and the Irish Sanctuary rid themselves of potentially the greatest threat the world had ever faced.
When Sebastian Tao and The Darquesse Society open a portal to the Faceless Ones' universe with the help of Silas Nadir, Sebastian goes through to see Darquesse fighting with the Faceless Ones. Neither side could win due to both having regenerative abilities. Darquesse had abandoned Stephanie (Valkyrie Cain's reflection)'s body in place of a magical form. When Sebastian asks Darquesse to come home, she follows Sebastian towards the portal, leaving the Faceless Ones behind.
At the end of the book, it is revealed by Valkyrie that the incomprehensible, mind-shattering forms taken up by the Faceless Ones are their "spirit state," and that their natural, physical bodies are "horrible and sickening and huge," but not incomprehensible to the point breaking the psyche. When she shows Omen the Faceless Ones that had already entered their reality, he describes one as "An impossibly huge thing, bigger than a mountain, with too many legs and too many eyes." They are depicted to be idle, standing, waiting, and that there are hundreds of thousands of them in total, some on Earth, but most still on the other side waiting for reality to be made habitable for them. The description suggests that the natural form of a Faceless One is not even vaguely humanoid, though their shape isn't described, it is suggested that they are chthonic and horrendous to behold.
While a direct description has never been given, Valkyrie described one as being five times taller than trees. Its appearance is said to not make sense and that it couldn't be real, as it had no weight or substance. It is said to have mass, but behind it is no depth.
However, while facing off against the Faceless One who had taken over Gruesome Krav, the narration indicates that the vessel the Faceless One had commandeered was the same size and shape as Krav had been before he was taken over - just without features. This is because they can only be observed by humans when they have taken over a body. It is possible that this is not completely accurate, however - a direct reference was never mentioned in the book. Because their true forms are abstract, and impossible for a human mind to comprehend, if someone looks at a Faceless One they lose their mind. This is the reason why no one knows what their true form looks like. Remus Crux glimpsed a Faceless One and went insane, as they can only be observed by humans when they have taken over a body.
When a Faceless One possesses someone that person becomes featureless: the person's hair falls out and their eyeballs, mouth and nose melt away, similar to Kith.
They are so evil that their shadows shy away from them.
As a race of immortal, lovecraftian esque entities, the Faceless Ones are beings of near limitless magical ability. They wield powers most beings could scarcely begin to comprehend and vastly eclipse almost every other magical entity shown in the series thus far.
They possess super strength, the ability to grow to the size of mountains, an enhanced healing factor, and the ability to fly. They are known to travel between realities and were powerful enough to escape being trapped by the Ancients in a barren dimension. They have shown that they are capable of extremely powerful feats of telekinesis, being able to pull apart or invert bodies without effort. The Faceless One that possessed Gruesome Krav turned Mr. Bliss inside out with a small hand movement.
These are only the powers that they've demonstrated but they should be, at the very least, capable of performing many more feats of magic as well as magic on a level similar to a sorcerer who has discovered their True Name.
The Faceless Ones are death and destruction incarnate. It's stated by Skullduggery in Dark Days that collectively they are powerful enough to decimate entire universes. They possess the power to destroy suns, lay waste to galaxies and kill basically anything in their path. Once all life had been snuffed from a universe they would move onto the next to start the process anew.
The Faceless Ones are also able to possess living creatures and in doing so cause their hosts eyes to melt out of their sockets, their nose and mouth to seal, their ears to melt away and all of their hair to fall out, resulting in a smooth featureless head. This, however, is not necessarily permanent as it is revealed in Bedlam, that if a Faceless One were to vacate its host before they burn them out the host in question would simply end up reverting back to their normal state. The only difference being an added strand of Faceless One DNA which 1 in 7 people in the present day are said to possess.
In their physical forms they are immune to harm from anything less than a God Killer weapon and can only be fought without one by a sorcerer who has discovered their True Name. During the Battle of Aranmore Farm they were shown to be completely unharmed by The White Cleavers scythe and during Playing With Fire Mr Bliss himself was unable to damage the Grotesqueries torso. Even after the Grotesqueries death its torso still proved impossible to destroy.
Although few feats have been shown in their true form it is hinted that they are just as powerful or perhaps even stronger without a physical vessel. Before the Ancients rose up against them The Faceless one didn't require physical vessels within this reality and could freely exist in their true forms. It was only after the Ancient made the environment of this reality to inhospitable for them that they were forced to take up physical vessels. This strongly implies that The Faceless ones were an even greater threat in their true forms as the Ancients appeared to have gone out of their way to force the Faceless Ones to take up a host. It is either that or they did this as a means to make the Faceless Ones vulnerable to the God Killer weapons. This conclusion is drawn due to the fact their true forms are shown to be formless, possessing no mass or substance and therefore making them impossible to damage via conventional weaponry. A God Killer weapon may be able to kill them but it can't kill them if it can't damage them.
It is known that a mere glimpse of a Faceless One without a vessel is enough to drive any normal sorcerer insane. (Remus Crux, Leibniz Eliza Scorn)
Throughout the series, and before, many followers of the Faceless Ones have tried to bring them back to the world, the most notable probably being Mevolent, who sparked War between the Sanctuaries and the followers of the Faceless Ones. All the followers of the Faceless Ones believe that if they bring the Faceless Ones back, the Gods would spare them and only kill those who didn't follow them. But some followers, such as China Sorrows and Jaron Gallow found out this wasn't true, and stopped following the Faceless Ones. Some followers of the Faceless Ones operate at the Church of the Faceless.
After Mevolent lost the War, Nefarian Serpine, a General of Mevolent's had tried to bring back the Faceless Ones by using the Book of Names to control someone who would know how to bring them back. After Serpine failed, another of Mevolent's general, Baron Vengeous tried to bring back the Faceless Ones using the Grotesquery (part of a Faceless Ones) to open a portal for them to return. However, Vengeous also failed, but only partly. The Grotesquery's death cry was able to alert the Faceless Ones the location of Earth.
Despite the disaster the Diablerie created, the Church of the Faceless still plans to bring the Faceless Ones back. Eliza Scorn, a follower of the Gods, is currently planning to bring the Gods back through unknown methods. Eliza has a list of twelve names that will help bring back the Church of the Faceless. She was also blackmailing China, until she revealed China's secret to Skulduggery and Valkyrie.