|Quoth the raven...|
The graffiti consists of a stylised raven with the word 'Nevermore' down its back, a reference to the Edgar Allen Poe poem 'The Raven'.
The stencil is signed 'SYD' - perhaps the artist? Searching online brings up a picture of an ant in a similar style on a wall in Hoxton, but no mention of the artist. Most mysterious... Whatever its origin, let's hope the painted version is less menacing than Poe's original.
If you have any answers about its origins, please leave a comment below!