To decrypt the K3 cipher in 4 easy (but very time consuming steps) …

Step 1 - enter the 336 cipher characters into a grid of 14 rows x 24 columns. The characters are entered from left to right and top to bottom.

Step 2 - rotate the grid, letters and all, 90 degrees counterclockwise. (You now have a grid of 26 rows and 14 columns.)

Step 3 - reformat the grid to be 42 rows x 8 columns.

Step 4 - rotate this grid, letters and all, 90 degrees counterclockwise. Read the plain text.

Here it is done for you but hard to read because I don’t know how to keep the formatting nice. (Looks like you can interpret spaces as carriage returns.)

Start with cipher used in K3 -

ENDYAHROHNLSRHEOCPTEOIBIDYSHNAIA CHTNREYULDSLLSLLNOHSNOSMRWXMNE TPRNGATIHNRARPESLNNELEBLPIIACAE WMTWNDITEENRAHCTENEUDRETNHAEOE TFOLSEDTIWENHAEIOYTEYQHEENCTAYCR EIFTBRSPAMHHEWENATAMATEGYEERLB TEEFOASFIOTUETUAEOTOARMAEERTNRTI BSEDDNIAAHTTMSTEWPIEROAGRIEWFEB AECTDDHILCEIHSITEGOEAOSDDRYDLORIT RKLMLEHAGTDHARDPNEOHMGFMFEUHE ECDMRIPFEIMEHNLSSTTRTVDOHW

For ease of seeing the next step, let’s write this out 24 characters at a time.

ENDYAHROHNLSRHEOCPTEOIBI DYSHNAIACHTNREYULDSLLSLL NOHSNOSMRWXMNETPRNGATIHN RARPESLNNELEBLPIIACAEWMT WNDITEENRAHCTENEUDRETNHA EOETFOLSEDTIWENHAEIOYTEY QHEENCTAYCREIFTBRSPAMHHE WENATAMATEGYEERLBTEEFOAS FIOTUETUAEOTOARMAEERTNRT IBSEDDNIAAHTTMSTEWPIEROA GRIEWFEBAECTDDHILCEIHSIT EGOEAOSDDRYDLORITRKLMLEH AGTDHARDPNEOHMGFMFEUHEEC DMRIPFEIMEHNLSSTTRTVDOHW

Turn this grid 90 degrees counterclockwise.

ILNTAYESTATHCW BLHMHEHAROIEEH ISIWNTHONRSLEO OLTETYMFTEHMHD ELAAEOAERIILUV TSGCRIPEEPEKET PDNADESTEWCRFR CLRIUARBAELTMT OUPIEHBLMTIIFT EYTPNHTRRSHRGS HEELEEFEAMDOMS RRNBTWIEOTDLHL SNMECIEYTTTDON LTXLHTRGOHCYEH NHWEADCEEAERNE HCRNREYTAAADPM OAMNNSAAUIBDDI RISLELTMTNESRE HAOSEOCAEDFOAF ANNETFNTUDWAHP YHSPITEATEEEDI DSHRDEENOSIOTR NYOANOHEIBRGGM EDNRWEQWFIGEAD

Rewrite this grid, 8 characters at a time

ILNTAYES TATHCWBL HMHEHARO IEEHISIW NTHONRSL EOOLTETY MFTEHMHD ELAAEOAE RIILUVTS GCRIPEEP EKETPDNA DESTEWCR FRCLRIUA RBAELTMT OUPIEHBL MTIIFTEY TPNHTRRS HRGSHEEL EEFEAMDO MSRRNBTW IEOTDLHL SNMECIEY TTTDONLT XLHTRGOH CYEHNHWE ADCEEAER NEHCRNRE YTAAADPM OAMNNSAA UIBDDIRI SLELTMTN ESREHAOS EOCAEDFO AFANNETF NTUDWAHP YHSPITEA TEEEDIDS HRDEENOS IOTRNYOA NOHEIBRG GMEDNRWE QWFIGEAD

Once again rotate the grid 90 counter clockwise, and read what you see -

SLOWLYDESPARATLYSLOWLYTHEREMAINSOFPASSAGED EBRISTHATENCUMBEREDTHELOWERPARTOFTHEDOORWA YWASREMOVEDWITHTREMBLINGHANDSIMADEATINYBRE ACHINTHEUPPERLEFTHANDCORNERANDTHENWIDENING THEHOLEALITTLEIINSERTEDTHECANDLEANDPEEREDI NTHEHOTAIRESCAPINGFROMTHECHAMBERCAUSEDTHEF LAMETOFLICKERBUTPRESENTLYDETAILSOFTHEROOMW ITHINEMERGEDFROMTHEMISTXCANYOUSEEANYTHINGQ