Travel to Stratholme and find Menethil's Gift. Place the Keepsake of Remembrance upon the unholy ground.
I watched as the human, Ras Frostwhisper, pledged his undying soul to the Lich King. He drew the dagger to his own throat and with a smile, cut from ear to ear. His body collapsed inside the borders of the pentagram. The Lich King stood over the fallen mage and with a single motion, Ras Frostwhisper the lich, was born.
<Leonid gazes at the ground and then looks up at you.>
Yes, [race], I remember. What you seek has a name - Menethil's Gift: The ground blessed by the Lich King - holy ground to Scourge.