What sacred object did the Philistines capture from Israel in battle?

Answer: The ark of the covenant

Israel brought the ark from Shiloh hoping it would save them, but the Philistines won, captured the ark, and killed Hophni and Phinehas.

“When the troops returned to the camp, the elders of Israel asked, “Why has the LORD brought defeat on us before the Philistines today? Let us bring the ark of the covenant of the LORD from Shiloh, so that it may go with us to save us from the hand of our enemies.””— 1 Samuel 4:3 (BSB)