T'Vala (Undine)

T'Vala was the adopted name of an Undine agent, real name unknown, who replaced the real Captain T'Vala of the USS Athens in 2409.

War Games
'T'Vala' was the leader of a group of Undine agents that replaced the entire crew of the starship Athens in 2409. The Athens was assigned to participate in war games, so T'Vala had the ship use real weapons and attempt to destroy the other Starfleet vessels. However, they lost the battle, and Starfleet disabled and boarded the Athens. They fought their way to the bridge, where T'Vala fought them in person after three security teams -- still in Humanoid form -- failed to stop them. T'Vala was defeated, even in Undine form, but escaped the ship. 

Deep Space Nine
T'Vala then joined the mission to infiltrate Deep Space 9. She hoped to use the station as a base of operations in the region, but Starfleet found out and retook the station. T'Vala fought to defend the station, in desperation, but was killed in the battle.