I am having trouble parsing and understanding Кого там нелёгкая несёт? Входите!
The context is that someone is interrupted by a knock on the door, and that is his response.
I am especially puzzled by нелёгкая. According to morfologija.ru this can be either an adjective or a noun (in the nominative). And the sentence seems to require that нелёгкая is a noun. However I can't find any English translation of the noun.
So what is нелёгкая, and what is the meaning of the sentence? Thanks!