What was this YA novel about an evil presence or cult in a cathedral?

Question

Let's see if I can be twice lucky today. I was given a children's or YA horror novel sometime in the '90s, which I actually never read as I found the premise too frightening at the time! Future Dan and his love for Lovecraftian horror scoffs at his past self. Here's what I remember:

I believe the book cover featured a looming cathedral, with a robed and hooded figure stood outside; from the robes, one could see a tentacle where the figure's arm should be.

The plot summary included a brother and sister; I think one of them fell under the sway of the evil force or cult in this cathedral, and their sibling had to save them.

That's all I've got. Let's see if anything hits!

Answer 1

While it does not match in every respect, I suspect you are remembering The Revenge of the Wizard's Ghost, the fourth Johnny Dixon novel by John Bellairs. Here are the front and back covers (illustrations by Edward Gorey):

The tentacled figure features prominently on a stained glass window from a chapel. It also appears in black and white on the frontspiece.

The main characters are Johnny and his two friends, the middle-aged Professor Childermass and Fergie. Johnny gets possessed by the ghost of the foe from the previous book, The Spell of the Sorceror's Skull, who was part of a family of evil magicians. This leaves Johnny an invalid for much of the book, until the curse is lifted by the efforts of the others, who need to recover two magical treasures from beneath the chapel on the villainous family's estate.