According to Exodus 12, Egypt's firstborn died in the final plague.
The event is presented as the climax of the ten plagues, after Pharaoh repeatedly refused to release the Israelites. The Israelite homes were marked with lamb's blood so the destroying force would pass over them. Egyptian households without that protection suffered the death of their firstborn children and livestock.
The story connects this plague with the origin of Passover. Exodus says that Pharaoh summoned Moses and Aaron during the night and finally ordered the Israelites to leave Egypt. The death of the firstborn is therefore not the same event as the Israelites' later crossing of the sea.
A common mix-up is to call this the plague of darkness or the plague of the firstborn's illness. The biblical account specifically describes death, and it identifies the firstborn of both people and animals as affected.