The Clathermonts

"Letters in and out of the city are routinely read. But one does not often intimately search a gentleman, and never a lady. So, the rumour has it, Clathermont and his three glum-faced tattooists - triplets, who some say are his daughters - although some hint at a more intimate relationship - hm - where was I? Ah yes. The rumour has it, spies come to Clathermont's to have messages tattooed in less public areas of their skin. This has, of course, been responsible for a scandalous vogue for secret tattoos."

Clathermont himself, as seen in SS.

Mr. Clathermont and his tattoo parlor, which is in the vicinity of Ladybones Road, are featured in both Fallen London and Sunless Sea. 

A tattoo from Clathermont's - a secret symbol of the Great Game.

Residents of Fallen London may get any number of small Great Game-related tattoos at Mr. Clathermont's parlor, provided one has enough connections to the Great Game and is sufficiently Watchful. During the Feast of the Exceptional Rose, residents can let Mr. Clathermont's wife Millicent give them tattoos if one has become sufficiently entwined in the family's intrigue and happens to possess an inspiration for the work.

By the way, Clathermont's triplet tattooists, mentioned in the snippets above, are named Edie, Myrtle, and Lily.