Inspired by the Fomorians of Celtic myth, they are depicted as humanoid, cyclopean creatures with barbed tails and beak-like snouts, with skins that varied from a dark green to a muddy brown colour. They are described as being part Daemon.
The Fimir inhabit the wetlands of the Warhammer world, typically within crudely constructed fortifications resembling nothing more than a pile of rocks. From these locations, the Fimir raid the homes of humans, taking captives for daemonic sacrifices. The Fimir loathe sunlight, and are followed by wreaths of mist to shield them from it, and their homes are always shrouded by it.
Fimir society is divided into a caste system, consisting of Meargh, the Dirach, the warriors, and the Shearl. The Meargh - also known as witch-hags - are the leaders of Fimir colonies, as well as the only females. A Meargh would typically also be a very powerful user of magic. The Dirach - described as "daemon-friends" - are a caste of wizards specialising in the worshipping of Daemons. The warrior caste - consisting of Fimm (warriors), Fianna Fimm (elite warriors), and the various nobles - take the brunt of raiding and fighting. The Shearls - the slaves of a Fimir settlement - exist only to work and die. The ruling Meargh hold the clans together. In the event of a Meargh's death, the Fimir of her stronghold separate, either working as mercenaries for other evil creatures or seeking out another clan to join. However the Meargh is sterile and therefore unable to breed. So as to replenish their numbers the Fimir kidnap young fertile human women to use as breeding stock.
The Fimir were created at the behest of Games Workshop's then-owner, Bryan Ansell who wanted a race "to be as distinctive of Warhammer as the Broo are of Runequest" . However, the Fimir did not prove popular, and disappeared with the 4th edition of WFB, although this did give them time to make a guest-starring appearance in Milton Bradley's HeroQuest. Very few (official) sources on Fimir exist and include the WFRP Bestiary, an article in White Dwarf No. 102, the third edition of WFB (Bestiary and Warhammer Armies) and some references in supplements.
There are two Fimir miniatures at the "correct scale" i.e. a size between 1.8m and 2.1m (in-game scale): Fimir (actually Fimm Warriors) from HeroQuest and some limited edition Fimm Warrior miniatures sold separately. One can also find Fimir by Nick Bibby the size of ogres, although these obviously do not integrate well with the "correctly" scaled models. Original concept sketches of the miniatures made by Jes Goodwin represented human-sized creatures and the published characteristics of the Fimir reflected this fact. However, when Nick Bibby started to sculpt the Fimir miniatures, he made them ogre-sized, leading to them being disproportionately weak for the size of their figurine in WFB. It was officially decided that this was the fault of the authors. Ultimately, WFRP was published with Fimir, but the race disappeared from the next edition of WFB.
Another theory as to the disappearance of the Fimir is due to the change in focus of GW's marketing. As 3rd edition drew to a close the focus became less on the older gamer and more towards the mid-to-late teens demographic, and as such it may have been thought that a race which reproduced via rape was not appropriate for the new target markets.
The last official appearance of the Fimir was in the WFRP sourcebook Marienburg: Sold Down the River, published in 1999. Since WFRP 2nd edition, the background has been brought into line with WFB and races such as Zoats, Gnomes and Fimir have been quietly dropped. Games Workshop described such races as being in "interminable hiatus". In response, several fan-written creations have been published, including a "Fimir special" issue 25 of the WFRP fanzine Warpstone.
There appears to be another reference to the Fimir in UK White Dwarf 310. Where in the article on the Gnoblar Horde, a new army list, there is a background piece on boglars, a sub-species of gnoblar that live in marshes. It says that "A large tribe in the Marshes of Madness have been rumored to be in alliance with strange, cyclopean creatures." While not being specific, this is almost certainly a reference to the fimir.
The Fimir were mentioned in the time of Legends novel 'Empire', where creatures matching their description were encountered living in the marshes.
Fimir are referenced by name in the "Marshland" entry of the terrain rules section of the 8th Edition Warhammer Fantasy Battles rulebook.
The July 2011 "Storm of Magic" supplement has, at last, reintroduced Fimir into the Warhammer tabletop game, in the form of a Dirach wizard called a Balefiend, which can be used as a bound monster by any Warhammer army. The character is now a full-fledged monstrous infantry (i.e. ogre-sized) model, rather than regular infantry.