Ensign Dern is a junior Enterprise-D officer who assists Worf with a test of upgraded photon torpedoes in Genesis, during which one torpedo veers off course.
Dern appears in the seventh-season episode Genesis. Worf has modified the ship's weapons systems and runs a live firing exercise with Dern working beside him on the bridge.
One of the torpedoes strays far from its target and has to be recovered before it becomes a hazard, so Jean-Luc Picard and Data set off in a shuttle to retrieve it. The errand keeps them away for several days.
In their absence a synthetic T-cell given to Reginald Barclay activates dormant genes throughout the crew, and the captain returns to a darkened ship whose people, presumably Dern included, have devolved into earlier forms of life.