Who is IS-K and where have they come from?

ISK has claimed the twin suicide attacks on Kabul's airport. Also known as ISIS-K or Islamic State Khorasan, who are they and where have they come from?

epa09430529 Smoke billows from the airport area after a blast outside the Hamid Karzai International Airport, in Kabul, Afghanistan, 26 August 2021. Source: EPA