Judges 11:39

וַיְהִי מִקֵּץ שְׁנַיִם חֳדָשִׁים וַתָּשָׁב אֶלאָבִיהָ וַיַּעַשׂ לָהּ אֶת נִדְרוֹ אֲשֶׁר נָדָר וְהִיא לֹאיָדְעָה אִישׁ וַתְּהִי חֹק בְּיִשְׂרָאֵל

It came to pass at the end of two months that she returned to her father, who did with her according to the vow that he had made; and she had not known any man. And it became a custom in Israel...

Yiftach (Jephthah) acted impulsively by offering the first thing to greet him as a sacrifice. Unfortunately, that turned out to be his own daughter. Nowhere, however, does it say that he actually sacrificed her, human sacrifice being detestable to G-d. The Radak tells us that Yiftach built his daughter a house, where he sequestered her away from society. The custom referred to, as continues in the next verse, is that each year other girls would come for four days to bewail her fate.