The Crimson Demon Clan is one of Crimson Demon mages who reside in their own village.
The Crimson Demons come from the same creator who made the Destroyer (being his preceding creations) and are mentioned in various Konosuba media through his diaries. The modification rights for the Crimson Demons were given as a prize in a gacha machine inside the Nosie Research Laboratory along with the prototype Playscation and Game Girl Color. In the past, humans of the kingdom he inhabited were recruited to be genetically modified to raise their level of magecraft. So many volunteered, however, that lots were drawn and the selection of subjects was heavily narrowed down. The surgery was a resounding success and the resulting new race had an extremely high aptitude for magic.
In regards to their chuunibyou tendencies, the people who were selected to undergo the operation all had this characteristic by complete happenstance. This resulted in their red eyes, strange names, and clan and race name, all a request by them to their benefactor.
A Crimson Demon's defining physical traits are their brown hair and red eyes, the latter of which will glow vibrantly when they're excited. They're extremely chuunibyou, evidenced by both names and demeanor. The race is extremely adept when it comes to anything related to magic, with adults all being archmages and possessing a repertoire of high-level spells.
Children begin attending the village's school at an early age and are only considered adults when they gain the necessary 30 skill points to learn advanced magic. To this extent, skillup potions are handed out as rewards for either demonstrating chuunibyou-esque behavior or knowledge of magic, both core tenets of Crimson Demon society. The other way skill points are gained is by power leveling, where the teacher of the class freezes the various dangerous monsters in the forest surrounding the village and the students kill them as they are subdued.
Past this, due to being a rather small village many residents have specialized tasks reminiscent of small communities pre-urbanization in both North American colonies and Europe during the 16th and 17th centuries. There is a shoemaker, a diviner (world-specific), etc, and even some village boys who hang around doing nothing. Children are expected to either become adventurers or succeed their parents in whatever job the latter performs. Similar to their other attributes resulting from chuunibyou-ness, all claim to be the "number one (job) in the village," or that they hail from the "number one (store) in the village," both of which are technically true as they are the only ones who do these jobs in the area.