This extraordinary looking creature is an oil beetle, to be precise a male Meloe proscarabaeus. (I looked that up: males have kinked antennae)
I’ve seen quite a number of them over the years in nearby natural settings. Today I am happy, having spotted four of them in my garden!
I’ll leave him in peace – I know better than to annoy an oil beetle, as they release a smelly, oily fluid when stressed.