Most of the Odd Cathedral is just Sneak and Combat checks, however there are 3 steps which require a specific character to clear. Check the log text to see if you're sending the right person, and you can click the Details button above the Log text (if there was a Sneak/Combat check), to see why/if you failed it.
Viewing post in Can't for the life of me find the secret ending
I completed the Odd Cathedral, but I'm confused as to what the point of the final expedition is if its not related to the secret ending. You get the key to the infested square in the second to last expedition, and the last one is just sending Scarlett to strangle the cult leader. My question is if I'm looking in the wrong place for the secret ending?