After completing the mission Horizon, speak with Miranda who asks you to go to Illium and help her rescue her sister from their father in the mission Miranda: The Prodigal. You have the option whether to complete this mission or not, the consequences of which can be seen below.
Choice: Complete Miranda’s Loyalty Mission or Not
All of your companions in Mass Effect 2 have their own personal mission which they ask you to help them with. Choosing to complete these unlocks their bonus power for use, gives a greater chance of them surviving the end of Mass Effect 2 and has repercussions for Mass Effect 3 if they survive.
Completing Miranda: The Prodigal
If you embark on this mission and successfully complete it, you will unlock Miranda’s bonus power Slam and give her a greater chance of surviving the final mission. If she survives into Mass Effect 3, and you warn her about Kai Leng, she will have a very good chance of surviving Priority Mission: Horizon.
If you have also completed Jack’s Loyalty Mission, you will be forced to intervene in an argument between Jack and Miranda, risking the loyalty of one of them depending on your current level of Paragon/Renegade points. See the Jack vs Miranda page for how to successfully keep Miranda’s loyalty.
Ignoring Miranda: The Prodigal
Should you ignore or skip this mission completely, Miranda will have a greater chance of dying in the suicide mission and even if she survives into Mass Effect 3, she will die by Kai Leng’s hand during Priority Mission: Horizon.