Sylvanas Impaled herself on the spires of Icecrown, So I wouldn't be surprised if the vision she saw was the afterlife/Shadowlands that Yogg-Saron controlled, which then lead to her getting some kind of Saronite poisoning in addition to her being scared of death until BfA were she' so delusional to think she can escape her fate or at least not suffer when she dies for good.