The Anti-Monitor was already there waiting for them. The Anti-Monitor told the Paragons that some fates are inescapable, that they may have stopped one version of Mar Novu from going through the experiment, but that another version of him did go through with. He then began to unleash his army of shadow demons.
Oliver then arrived, and told them to listen to him, that this is why they were selected as Paragons. Oliver said that being Paragons means they are all powerful enough to hold the line; to buy them the time they need to rebirth the universe. He explains that the dawn of time is the only place where this do over can happen. He goes onto say that he will light the spark, but they need to fan the flame. When Lex asks how they will do that, Oliver replies that they'll know when it's time.
The Anti-Monitor tells Oliver that it's too late and that after 10,000 years he will finally be victorious. Oliver says that no he won't win, and that he believes his entire life has prepared him for this fight. The Anti Monitor says that all life ends, and that only death is eternal.
When he says that only he is eternal, Oliver takes off his hood, and tells the Anti-Monitor that "we'll see about that."
Oliver and the Anti-Monitor engage in a fight, and as they're fighting, Oliver tells hims that he has failed this universe and unleashes a surge of power.
Their clash provided the spark, and it was up to the Paragons to fan the flame. They focused on Hope, Destiny, Humanity, Love, Truth, Courage, and Honor. The resulting blast makes the Anti-Monitor disappear. With Oliver dying, Barry brings Sara to him, and he makes both of them promise to watch over his family, as he passes away.