The Adventure of the Deadly Dimensions — Lois Gresh

THE SHAPE OF ANCIENT EVIL

A series of grisly murders rocks London. At each location, only a jumble of bones remains of the deceased, along with a bizarre sphere covered in strange symbols. The son of the latest victim seeks the help of Sherlock Holmes and his former partner, Dr. John Watson.
They discover the common thread tying together the murders. Bizarre geometries, based on ancient schematics, enable otherworldly creatures to enter our dimension, seeking to wreak havoc and destruction.

The persons responsible are gaining so much power that even Holmes's greatest enemy fears them -- to the point that he seeks an unholy alliance.