SPOILER WARNING: This article contains major spoilers for "11:59," tonight's episode of "Arrow."
Since the season four premiere, "Arrow" has teased that a main cast member would die. In "11:59," the latest episode, the show made good on that promise and killed Laurel Lance, aka Black Canary.
Throughout the episode, Oliver and Dig butted heads over whether or not they could trust Dig's brother Andy. As it turns out, Oliver was right, and Andy was working for Damien Darhk all along. Between Andy and Malcolm Merlyn, Darhk was able to restore the idol that grants him his powers and broke out of prison. Team Arrow converged on the prison to stop him, but they were too late. Darkh easily stopped them and revealed that he knew their secret identities just before he made good on his promise to kill Quentin's daughter by shoving an arrow into Laurel's chest.
She initially survived the blow, but the rest of the team stayed at the hospital to see her through recovery. She regained consciousness with enough time to tell Oliver that she wishes the best for him and hopes he can find his way back to Felicity, just before she suffered an embolism, which ultimately killed her.
Once he heard that Black Canary was hospitalized, Quentin Lance rushed to the hospital. However, because he provided a distraction in order for Team Arrow into the prison, he was arrested and this delayed him. Laurel passed away before he made it to the hospital.
Additionally, the teaser for the next episode confirms Laurel is in the one in the grave.