なり

「なり」is used to describe a situation where one action (V) immediately follows another action, often unexpectedly, right after the first. The subject (agent) of both actions is usually the same and is often in the third person.
It's important to note that the second clause does not pair with structures indicating intention, command, prohibition, or negation.
It is synonymous with「や否や」(ya ina ya) and「たとたんに」(tato tan ni). However,「なり」and「や否や」are more commonly used in writing, while「たとたんに」is more commonly used in spoken language.