Who are the Wagner Group, and why are they in Ukraine?

According to media reports, Ukraine's president, Volodymyr Zelensky, has survived two assassination attempts from the band of mercenaries known as the Wagner Group. Their ruthlessness has earned them a feared reputation from Kyiv to Central Africa. But who are they, and has Putin really entrusted them with taking out a head of state? (Pro-Russian separatists patrol with armoured vehicles in Donetsk, Ukraine 11 March 2022. Getty Images)