public class QuestStep
{
    public static List<QuestStep> QuestSteps = new List<QuestStep>();

    public int QuestID;
    public int TaskID;
    public int StepNumber;
}

public class QuestTask
{
    public static List<QuestTask> QuestTasks = new List<QuestTask>();

    public int ID;
    public string TaskName;
}

public class QuestInfo
{
    public int ID;
    public string QuestName;

    public List<List<QuestTask>> TaskSteps = new List<List<QuestTask>>();
}

Linq query to fill the TaskSteps in QuestInfo.
The outer list should be a descendingly sorted list(by stepnumber) of Lists with QuestTasks, that correspond to the QuestID(from QuestStep)