expression, noun-modifying (irregular)
Used as a prenominal phrase to describe something that really happened or exists, often in contrast to fiction or rumor. Common in phrases like 実際にあった話 (a true story).
See also: 実際
これは実際にあった話です。
This is a true story.
実際にあった出来事をもとにした映画です。
It's a movie based on actual events.
実際 is a noun/adverb meaning 'reality' or 'actually', while 実際にあった is a fixed phrase used attributively to mean 'real/actual (event/story)'.
Phrase composed of 実際 (actuality) + に (adverbial particle) + あった (past form of ある 'to exist'). Literally 'existed in reality', used as a set phrase.