Transphasic torpedoes were weapons brought back from the future by Admiral Janeway, able to destroy a Borg cube with a single hit.
An older Kathryn Janeway, who had spent decades watching the consequences of Voyager's long journey home, travelled back in time with technology designed to change the outcome. Among it were transphasic torpedoes, whose warheads had been designed so that Borg shielding could not adapt to them.
Voyager used the torpedoes to devastating effect against Borg cubes during its final confrontation with the Collective, and the weapon was a key part of the plan that brought the ship back to Earth while dealing the Borg a serious blow.