adverb
suddenly; abruptly; all of a sudden
Describes a sudden, often dramatic change or occurrence. Common in narrative and descriptive contexts.
空が俄然暗くなり、雨が降り出した。
The sky suddenly darkened and it started to rain.
彼の態度は俄然変わった。
His attitude changed abruptly.
adverb
Colloquial intensifier, often used to emphasize a superlative or a stark contrast. More common in spoken or informal written Japanese.
この店のラーメンは俄然うまい。
This shop's ramen is by far the best.
彼はクラスで俄然足が速い。
He is absolutely the fastest runner in the class.
突然 is the most common word for 'suddenly' and is neutral in register. 俄然 carries a stronger nuance of a dramatic or striking change, and also has a colloquial intensifier use that 突然 lacks.
急に is a very common, casual way to say 'suddenly'. 俄然 is less frequent and more emphatic, often implying a noticeable or surprising shift.
The word is composed of 俄 (sudden, abrupt) and 然 (a suffix indicating state or manner). The exact historical derivation is uncertain, but the combination has been used since classical Japanese to express suddenness or emphasis.