Note: this answer is all spoilers, taken from the wiki page:
If you spared the Rachni queen in the original Mass Effect, the Reapers find it and indoctrinate her children, although not herself.
Shepard has a choice to either save the queen (sacrificing Aralakh Company) or let her die (saving Aralakh Company). If she is saved, Aralakh is slaughtered, and Grunt makes a last stand holding off the rachni Husks called Ravagers. If Grunt wasn't loyal to the mission, then he will die, but if he was loyal, he will survive. If she is saved, rachni workers are sent to the Crucible project, where they are prized assets for their engineering capabilities, industrious nature, and good teamwork.
If Shepard did not spare the Rachni queen on Noveria, the Reapers will artificially construct a Queen thrall in order to control a rachni army. Shepard can choose to spare this artificial queen even if the Commander killed the real queen. This queen will eventually betray Shepard, taking the Rachni Workers War Assets away and decreasing the numerical values of some Alliance fleet War Assets.
