Who are ISIS-K?
ISIS-K is an affiliate of the so-called Islamic State terrorist group, which once gained large territories in northern Syria and Iraq.
The splinter group, which was established in 2015, has been mostly based in eastern Afghanistan, part of an area known as the Khorasan province - which is referred to in their name ISIS-K.
Back in 2017, the United States dropped what was known as "the mother of all bombs" in the area to try and send a warning and eliminate the threat.
However, its fighters are thought to number up to 2,200 - although this figure could well be rising given the security vacuum left by departing foreign troops from Afghanistan.
https://www.itv.com/news/2021-08-26/who-are-i-k-the-islamic-state-group-posing-threats-to-afghan-evacuations
Islamic State Khorasan (IS-K) is the Islamic State’s Central Asian province and remains active three years after its inception. The Islamic State announced its expansion to the Khorasan region in 2015, which historically encompasses parts of modern day Iran, Central Asia, Afghanistan, and Pakistan.
https://www.csis.org/programs/transnational-threats-project/past-projects/terrorism-backgrounders/islamic-state-khorasan