Drack sent a strange email, asking you to meet him on the Nexus. He won't say why.
This mission can be acquired after completing "Epilogue: Home and Away", and by acknowledging an email found in your email terminal titled "Some good news". The alternative is to just skip the email and go to Drack directly on the Nexus, as described in the walkthrough.
Read email from Drack (Optional) Edit
This is simply done by acknowledging the email titled "Some good news" at any email terminal on the Tempest.
Meet Drack on the Nexus Edit
You will need to find Drack on the Nexus, which is pretty easy. Go to the second level of the Docking Bay, and head towards the Nexus apartments. You will want to open the first apartment on your left, and the next scene will play with you walking into Dr. Carlyle telling Drack and Kesh that everything is going well. When you step in, he will ask if he can watch the secret being revealed to Ryder. From there, you will be able to respond accordingly to the "secret".
After this short dialogue, Kesh will explain that they're Krogan children, and that this is very exciting for them. Drack responds by saying that they are anxious for the future, but that Ryder will be included in that future. Depending on how you respond, Kesh and Drack will either respond saying that you won't be the only godparent, or by saying that Drack will teach the Krogan children everything about what it means to be a Krogan, just like Kesh was taught. The mission is marked as completed at this point, but there is one more step...
Once you return to the Tempest, you will see another email from Drack titled "Future out here". Give it a read, and prepare to witness the closest anyone's ever been to seeing a krogan cry.