Back in the late 80's to early 90's I read a strange book about a family from Castle Inverness; they have defended the earth from demons invading for centuries (I'm 99% sure "Inverness" is in the title; but I can't find anything online; it's either Castle Inverness or Return to Inverness or ??). I'm pretty sure it's a series but I only read the first book.
The "lord" of the castle is bed ridden; technology can only be used briefly in the castle to check on him (i.e. with a flash light). Later you find out the the old man is alive; his soul is trapped in the magic realm of swords and sorcery.
There are many characters in the story. I think there were two sons. One son tries to rescue the father (the lord of the castle) but he is tricked into helping the main villain; who is now free and the son is imprisoned in a cage of spikey bones (I think) hanging at an impossible height.
The other son didn't believe in magic and left the castle years before; but he returns to be with his father and eventually realizes that magic and demons are real. At the end of the novel, he magically wields Thor's hammer, he throws it twice to fight the enemy and it breaks both of his arms (upon catching it).
I also remember a married couple who were involved somehow. At the end of the first book, the husband wields the Ring from Wagner's Ring cycle (to fight the demons) and his wife abandons him. The wife is actually a fairy and she remembers who she really is and she flies off because her husband did something terrible to defend her earlier in the story.
There was also a mildly humorous section about the demons recruiting ignorant and disgruntled humans to help them fight against the heroes. These humans had no idea what they were signing up for and eventually they would wind up as skins for the demons to wear.
I vaguely remember that the cover of the book depicted a storm over a castle and I think there may have been a knight swinging a sword at the storm.
Does anyone know what this series is?