The Roman Candle. Bone Guard, #6

A hidden masterpiece points the way to a treasure lost since the fall of Rome! Before fleeing her Mossad pursuers, his Nazi client, the Warden, leaves Grant Casey a final gift-or is it a curse? An unknown Caravaggio, coveted by collectors, and hunted by the man who chained Grant to a sinking ship and left him for dead. When the painting goes missing, an ambitious art restorer will do anything to gain access to the lost work-even if it's over Grant's dead body. Renaissance science and the artist's tragic life and untimely death provide the clues, while team member D.
A.'s past rises to haunt the hunters. The last thing Grant wants is to get involved, but if his enemy wins the race, it's literally the beginning of the end:  the flame to inspire the Apocalypse.