The 2401 death of Commander Ro Laren, who gave her life fighting the Shrike so that the USS Titan could escape.
Ro Laren, a former Enterprise officer, served in Starfleet Intelligence and had been investigating the Changeling infiltration. In 2401 she met Picard aboard the USS Titan and passed on what she had learned, including how deeply the Changelings had reached into Starfleet.
When Vadic's Shrike closed in, Ro flew her own ship against it to buy the Titan time. The Shrike destroyed her ship and she was killed. Her evidence helped Picard and his allies understand the threat they faced.