Narcissa didn't even really notice when the pain potions began to take immediate effect. They pain was so intense that her body seemed to have made her feel numb to it as protection. All she could focus on was the fact that she needed to tell Lucius, she needed to tell him before she slipped under in case she never woke up again. She squeezed her hand weakly, though it took a great amount of effort.
"I love you," she whispered, not noticing that her sister had arrived just in time to hear her declaration. "I love you and only you, Lucius."
And then the potions, and the blood loss, and the unacknowledged pain pulled her under.