Fantasy novel about a monster infecting a kid and a bunch of magicians have to fight it off.

[Collin]

I probably read this book from 2005-2010, and the plot seems very vague and abstract in the long while since I've read the book. Here is the plot as I remember it although it is jumbled.

This book takes place during modern day and our main character is a young kid who finds out his grandpa(?) is part of a magicians society and so are the grandpa's friends. At some point the main character has a dream about a stone platform or circle detailing that something evil is there or lives there. Some sort of smoke monster or evil thing made out of rags? He tells his grandpa and he takes his magician buddies out to this stone place and someone casts a time lapse spell to see if anything bad has happened in the past to the place. Turns out nothing bad happened and they all go on to live their lives. At some point this smoke monster or mummy or whatever stalks the kid and infects him? I don't remember this too well, I know that the grandpa dies and that the book ends with some musing about a riddle that the old man left behind.

One last thing, I remember the cover of this book looking like a tornado, tapering towards the top, with a skull looking down on a guy pointing up at it? Very hazy memory.

I appreciate all your help to try to find this book, fair warning I do not have high hopes. I probably read thus book when I was like 6 or something so everything I remember is probably very off.