The Challenge from Beyond

George Campbell sets out with a tent into the Canadian forests to find respite from work. There, he comes across a crystalline cube that exhibits extraordinary properties – the longer one gazes at it, the more it glows, beginning to hypnotize the observer. Campbell is reminded of the story of the so-called Eltdown Shards, whose alleged translation spoke of crystalline cubes and a race of entities sending minds into the Universe for exploration using such cubes.
The creator of the story was the young Julius Schwartz, publisher of the fanzine "Fantasy Magazine," who asked a group of well-known writers to create a multipart story based on the given title - each writer was to write one segment. One group of authors was to create a work in the spirit of science fiction, while the rest (including Lovecraft) - in the spirit of supernatural or fantasy tales.
C. L. Moore started the story. Then, Frank Belknap Long wrote his part. The next in line, the highest-paid writer among them all, A. Merritt, declared that Long had deviated too much from the title's essence in his development of the story. Merritt set a condition: either Long's portion would be rejected and replaced with his fragment, or he would withdraw from the project. Schwartz couldn't afford to lose such a prominent name, so he thanked Long. Of course, Merritt's contribution was minimal, and to advance the plot, Lovecraft had to intervene, creating the longest fragment of them all. The rest was completed by Robert E. Howard and – convinced by H.P. Lovecraft to return – Frank Belknap Long.