Midnight Society - The Black Lake (2016)

Forty years ago, England's greatest adventurers shattered their friendship, making a terrible choice that would come to haunt humanity. Forty years later, when she's sent on a desperate rescue operation to Scotland's Loch Ness, secret agent Matilda Finn faces the consequences of that choice and questions her already-loose ties to human society.
Collects Midnight Society: The Black Lake #1 - #4.