A thick, tough brain-like jelly fungus.  The hymenium (fertile surface) has fine warts.   Colour varies from a translucent brownish grey to black, and it shrivels to a brownish crust when it dries out, regaining its shape and jelly-like texture when re-wetted. 

Very similar to Witches' Butter, Exidia glandulosa, but this does not form brain-like fruit bodies.  They are 'microscopically indistinguishable' (Roberts, 2001)

Photograph in habitat; provide close-up photos of upper and lower surfaces; note habitat, substrate and associated tree species.  It can be difficult to separate Warlocks' from Witches' Butter and close examination of the growth form and the texture of upper and lower surfaces is needed.

On dead wood, especially beech, hazel and ash; very occasionally oak

https://www.first-nature.com/fungi/exidia-nigricans.php