Who are the Wagner mercenary group being led by ‘Putin’s chef’?

n Ukraine, Russian soldiers haven't taken a town or city for months; but in the last 24 hours a private army - pro Russian mercenaries - claim they have, right next to the Donbass region in the east, in some of the bloodiest, heaviest battles of the war so far.

These men are not formal troops trained and led by the Russian military, but ones who make up a notoriously brutal collection of fighters called the Wagner Group, led by a rich Russian businessman Yevgeny Prigozhin who used to be known as President Putin's chef.

So who are this shadowy army and what exactly is their relationship with the Kremlin?