Lieutenant Edward Hagler is an Enterprise-D officer taken by the solanogen-based aliens in Schisms, who is returned to the ship only to die when his blood turns to a liquid polymer.
Hagler figures in the sixth-season episode Schisms. As the senior staff realise that crew members are being removed from the ship in their sleep, the computer reports that two people are missing at that moment: Ensign Rager and Lieutenant Hagler.
Hagler reappears soon afterwards in a state of collapse. Beverly Crusher finds that his blood has been transformed into something resembling a liquid polymer, and he dies in sickbay before anything can be done for him.
His death shows that the experiments being carried out in the subspace domain are lethal, and adds urgency to the plan for William Riker to stay awake, let himself be taken and bring Rager back before the rupture in the cargo bay is sealed.