"It means that fear has taken a physical form......and now, it's trying to suck people into Madness directly...the Madness must be getting more contagious. That's the only explaination..."
A Clown, also known as a Pierrot, is a being which is the very embodiment of the contagion of Madness and the physical form of fear itself. Clowns are created due to the spread of the Madness of Fear from the Kishin Asura's Madness Wavelength. The Clowns later form the Clown Army to protect Asura, though they're later killed by the combined might of the DWMA and the Witch Order.
Despite the uniformity in each individual Clown's purpose and ability, their actual appearance varies greatly between them, with the only visible exception being that they all possess a vaguely humanoid structure in their stature. The remainder of their bodies are seemingly composed from a random assortment of items, which is as distinctive as each particular Clown.
After an ample amount of time has passed in which Madness becomes rampant, there are two processes in which Clowns are created. One process requires the use of a factory such as the Borscht Seven Factory, which is a crib of Madness and is used to create Clowns. Another method is possible in which another Clown can reassemble the torn and burned remains of previously killed Clowns, reforming them into orbs topped with flames. The orbs then lift to reveal stalks underneath them digging into the surface, eventually forming into another Clown.
Main article: Man-made Clown
Although each Clown has its own individual and unique abilities (including even Magic), all Clowns possess the ability to regenerate from death from the usage of the abundant output of the Madness of Fear from the Great Old One, Asura. Clowns also have the common ability of utilizing Madness Infection.
Clowns also are capable of joining with other Clowns or with humans. One such technique, Madness Fusion, allowed Justin Law to combine with his Clown. Another technique allowed Kaguya, Moonlight, and White Rabbit to merge by first dissolving their bodies into their souls, then combining them before their merged bodies formed around the combined souls.
While Clowns themselves can still be killed, with another present means regeneration is possible and can cause a battle to become endless. According to Death, one way to eliminate this is to rebuff the Kishin's Madness Waves from within a Clown to cut the cycle of Madness and stop the regeneration. This is possible with the Soul Protect spell allied as "Protect Bullets". However, Eruka Frog stated such a feat isn't possible with one or two witches and requires a team.
In addition, though not explicitly mentioned, the Madness from Asura's Wavelength, which the Clowns feed off of, could potentially be blocked by the Madness of Order due to the fact that even Asura states the unique Madness from a True God of Death can counteract his own.