The Glowing Sea

Make preparations like stocking up Rad-X and Radaways as well as high radiation resistance equipment and power armor. Once done, make your way to the Crater of Atom and talk to Mother Isolde. She'll point you to the Rocky Cave in the far southwest of the Glowing Sea. There's a deathclaw resting outside the cave so it's up to you to sneak behind it or kill it.

Head inside and talk to Virgil. Agree to help him and he'll discuss the details. You'll learn that you need to kill a courser and receive instructions from Virgil. Once you're done talking, the quest will be complete and next one starts immediately.