What's the Name of That Book??? discussion

A boy/young man goes to Wales for some time to live with his aunt.
On the train he sits next to a female professor. Somewhere around the trip there's an old hag who's been following the boy around, she starts wailing. The boy thought he was alone to see her, but the professor can hear the hag, too.
When he gets to the aunt's cottage, there's no-one there. The professor lives nearby. She has a collection of masks and somehow the boy can see they are alive.
There's another story about an Elizabethan magician who summoned the old hag, though she wasn't old then.
This magician possesses the boy's aunt.
There's also an isolated mansion involved where lives an old man and his daughter with a mermaid.
The magician tries to kill everyone.
The book ends in Canada, I think, with a shaman walking in to a house in the middle of the night, and only a young girl can see her.

This is a relatively new book, published in this century, possibly even this decade, and it was supposed to be first in a series.

Ketutar, what year did read you read this book?

Is this a novel or a book with a number of short stories (how many)?

I'm not sure when I read it, 2007? 2014? 21st century, any way. I can't get any closer than that, except that it was some years ago.

It is a novel, first part of a series. So it's not three, four different stories, but parts of the story.

The boy is the main character, the story about the magician is a parallel, or frame story. The magician performs some demon magic to put his life into something so that he can live forever, and the boy's aunt figures this out and releases the magician, who then possesses her and starts causing very bad things.
The magician also summoned the old hag, who wasn't an old hag then, but the most beautiful woman ever, Cassandra of Troy, and they loved each other. Cassandra has been walking the earth since then, and is at the time of the events in the book an old, old woman. The boy is the only one able to see her, and she is stalking him, because she knows things about him.
The professor gives the boy a lift to his aunt's cottage, because she lives nearby, and because the aunt is nowhere to be found, the boys ends up sleeping at the professor's house, and then he sees the masks and finds out they are "alive".
One of the masks, a wolf or hound or something else canine, becomes a real animal, who kills Cassandra.
The mansion is also nearby the aunt's cottage, and the aunt is a friend of the people living there; an old man and his daughter who is special.

Advent by James Treadwell?

One Amazon reviewer says, "Advent is a retelling of the legend of Faust.The book really only uses it for some main facts, mixing it with the legend of Cassandra and Helen of Troy. Advent alternates between the story of Gavin, an only child who has always been a bit different and the greatest magister (magician/warlock) the world has ever known Johannes Faust. Treadwell switches between the two, going backward and forward in time until their two stories converge, with very unpleasant consequences."

Google search -- site:www.goodreads.com "young adult" "fantasy" "Cassandra of Troy"

Yes! That's it! Thanks, Kris.

My google search results with those words:
Antigoddess
Cassandra
The Firebrand
Inside the Walls of Troy
Mogsy
The Moon Riders... :-D

You're welcome! Today the results of that Google search turns up Advent right after "The Moon Riders" on my computer. You never know what you'll see, partly depending on your geography. I took a chance using your unique phrase "Cassandra of Troy" even though she might not appear in the main blurb - but in someone's book review.