If a group of people descend on a place or person, they arrive, usually without warning or without being invited.

Sorry to descend on you like this, but we didn't have a phone to call you. 如此唐突造访非常抱歉，我们没有电话，不能提前通知你们。

The police descended on the house in the early hours of the morning. 凌晨警察突然闯进那所房子。