Learn Danish: Hendes værste frygt gik i opfyldelse.

Possessive pronouns like hendes make the noun phrase definite. In Danish, adjectives following a possessive must take their definite form, which almost always ends in -e. That is why we use værste instead of the basic form værst.

The translation says "came true", but what is the literal meaning of gik i opfyldelse?

Literally, it translates to "went into fulfillment". Notice how Danish uses the past tense of at gå (to go) for this idiom, whereas English uses "to come". The noun opfyldelse means fulfillment or realization.

How is the word opfyldelse formed, and can I use its root in other ways?

It is formed from the verb at opfylde (to fulfill, satisfy, or meet) plus the suffix -else, which turns verbs into nouns—much like "-ment" or "-tion" in English. You will often see the root verb used in phrases like at opfylde kravene (to fulfill the requirements).

Could I use angst instead of frygt to mean "fear" in this sentence?

Not naturally. Frygt is the standard word for a specific, identifiable fear or dread. Angst translates closer to "anxiety" or a deeper, more general psychological dread. For a specific fear coming true, værste frygt is the expected pairing.