Campbell wanted to publish a fantasy magazine with more finesse and humor than Weird Tales, and put his plans into action when Eric Frank Russell sent him the manuscript of his novel Sinister Barrier, about aliens who own the human race.

Campbell required his authors to avoid simplistic horror fiction and insisted that the fantasy elements in a story be developed logically: for example, Jack Williamson's Darker Than You Think describes a world in which there is a scientific explanation for the existence of werewolves.

Similarly, L. Sprague de Camp and Fletcher Pratt's Harold Shea series, about a modern American who finds himself in the worlds of various mythologies, depicts a system of magic based on mathematical logic.

Other notable works included several novels by L. Ron Hubbard and short stories such as Manly Wade Wellman's "When It Was Moonlight" and Fritz Leiber's "Two Sought Adventure", the first in his Fafhrd and the Gray Mouser series.

[3][4] In the meantime, science fiction was starting to form a separately marketed genre, with the appearance in 1926 of Amazing Stories, a pulp magazine edited by Hugo Gernsback.

In 1930 pulp publisher Clayton Publications launched Astounding Stories of Super Science,[5] but the company's bankruptcy in 1933 led to the acquisition of the magazine by Street & Smith.

[6] The title was shortened to Astounding Stories, and it became the leading magazine in the science fiction field over the next few years under the editorship of F. Orlin Tremaine.

When Eric Frank Russell sent him the manuscript of his novel Sinister Barrier, Campbell decided it was time to put his plans into action.

[11] When wartime paper shortages became severe in late 1943, Campbell chose to keep Astounding monthly and cancel Unknown, rather than switch the former to a bimonthly schedule as well.

[9] The first issue, the following month, led with Russell's Sinister Barrier,[note 1] the novel that had persuaded Campbell to set his plans for a fantasy magazine into motion: the plot, involving aliens who own the human race,[9] has been described by SF historian Mike Ashley as "a strange mixture of science fiction and occult fantasy".

[12] Campbell commented in a letter at the time that Sinister Barrier, "Trouble with Water", and "'Where Angels Fear ...'" by Manly Wade Wellman were the only stories in the first issue that accurately reflected his goals for the magazine.

[13][note 2] Under Campbell's editorial supervision, the fantasy element in Unknown stories had to be treated rigorously.

[9] De Camp, in collaboration with Fletcher Pratt, contributed three stories featuring Harold Shea, who finds himself in a world where magic operates by rigorous rules.

[21][note 3] SF historian Paul Carter has argued that the spectrum of fantastic fiction from Weird Tales through Unknown to Astounding was far less cleanly separated than is sometimes assumed: many stories in the early science fiction magazines such as Wonder Stories were more like the works of Edgar Allan Poe than they were tales of scientific imagination.

The protagonist, a university professor, "is forced to abandon scepticism and discover the underlying equations of magic, via symbolic logic", in critic David Langford's description.

[17][23] Other notable stories that appeared in Unknown include Jack Williamson's "Darker Than You Think" (December 1940), which provides a scientific basis for a race of werewolves living undetected alongside human beings.

Expanded into a novel in 1948, it remains Williamson's best-known fantasy, and SF historian Malcolm Edwards comments that the two protagonists' relationship is "depicted with a tortured (and still haunting) erotic frankness unusual in genre literature of the 1940s".

[12][17] The cover art came almost entirely from artists who did not contribute to many science fiction or fantasy magazines: six of the sixteen paintings were by H. W. Scott; Manuel Islip, Modest Stein, Graves Gladney, and Edd Cartier provided the others.

Cartier was the only one of these who regularly contributed to SF and fantasy periodicals; he painted four of Unknown's last six covers before the change to a text-heavy design.

[29] Edwards comments that Unknown "appeared during Campbell's peak years as an editor; its reputation may stand as high as it does partly because it died while still at its best".