In the discussion with Andreas Eriksson on the X-Band: The Phantom Podcast episode 201 he mentioned this book 5 months ago!
There he mentioned it was planned to be released within the first half of the year.
It has also been hinted at in a Fantomen comic in the beginning of the year.
But now we finally know what this secret project is thanks to Kristian Hellesund, who pointed us to a post on Facebook where someone had collected all the upcoming comic book titles for 2022 and this "Heart of Darkness" clue was there.
The long story was first published in 1994-1996 with a few pages in each issue of Fantomen / Fantomet / Mustanaamio comics.
It then got reprinted in Fantomen krönikan in a more collected form that span 4 full issues and added a new story of 38 pages. You can now pre order it from Aldibris.com here, the price is 564 kronas ($60US, €55 & $80Au).
"The Heart of Darkness" is a 390 pages long Team Fantomen series written by Claes Reimerthi and drawn by Joan Boix. The story features the Phantom facing several supernatural elements, and ran as a backup story in Fantomen from 1994 to 1996".
The story also makes some references to the 1985 Team Fantomen story "The Fire of Life", that is 32 pages.
This book will probably be between 360 and 430 pages depending on what they include. A real brick of a book in other words.
Thanks also to Kristian Hellesund for letting us know about the announcement.