Suddenly; all of a sudden; unexpectedly—often used in formal or written contexts.
突如、会場が静まり返った。
Suddenly, the hall fell silent.
彼は突如会社を辞めると言い出した。
He suddenly announced that he would quit the company.
空が突如暗くなり、雷が鳴り始めた。
The sky suddenly darkened and thunder began to roll.