What is Wagner Group, the Russian mercenaries said to be in Kyiv to kill Ukraine Prez?

Mercenaries from Wagner Group, a Russian private military company recently sanctioned by EU over human rights abuses, are reportedly in Kyiv to assassinate Ukrainian President Volodymyr Zelenskyy. The company is believed to be used by Kremlin to carry out its strategic objectives, even though Russia denied such claims. Wagner was reportedly involved in beheading of Syria army deserters in 2017.