Blake's Laocoön engraving is unique within his oeuvre because it is an imaginative work whose central feature is directly copied from another work of art: the Vatican Laocoön group. In commercial engravings Blake was of course required to copy on a regular basis, but in his creative pieces Blake always adapted borrowed figures to their new surroundings. In no other work does he draw attention to the copy as he does in the Laocoön. The copied image is highlighted moreover by another unusual feature of the engraving: Blake often combined written and graphic art, but, whereas he more commonly surrounded his writing with images, here the image is surrounded by writing.
I have published elsewhere on the connection between this engraving and Blake's commision for Abraham Rees' Cyclopaedia ('Encyclopaedic Resistance: Blake, Rees's Cyclopaedia, and the Laocoön Separate Plate') and on the connection between this engraving and the Napoleonic art confiscations of the 1790s ('William Blake, Empire and the Napoleon Factor: Rethinking Empire and the Laocoön Separate Plate').
I remain indebted to Robert N. Essick for granting me permission to use the image of his copy of this incredible engraving.