Chapter 9: Rising Action - Borderlands 2
Things will get messy as soon as you pop that Hyperion Power Core in so before doing that, I suggest completing these optional missions:
Mine, All Mine – Lilith
The Pretty Good Train Robbery – Tiny Tina (after Mine, All Mine)
The Good, The Bad and the Mordecai – Bulletin Board
Once you're ready, head to the shield generator and remove the old power core. Replace it with the Hyperion core you acquired earlier and all hell breaks lose. Head to the town center and meet up with Scooter. Head to the ignition primer and cycle it as instructed.
Next, head to the HQ and meet up with Roland. Grab all five Eridium nuggets and deliver them back to Lilith. Watch the next scene then leave the Sanctuary.
Once done, catch a ride then leave to the Three-Horns Valley. Find the hill leading to the Fridge's entrance then activate the button to turn in the mission.