Scene summaries may contain spoilers
The portal opens into the empty Roman encampment, meaning the troops have already begun to march. If you don't catch up with them before the fighting breaks out, it'll be too late to save them.
Sam thinks the troops are heading towards the valley filled with wildflowers, which Maeve says is where her people celebrate the coming of spring. She is filled with righteous indignation at how the Romans have treated her people, and doesn't know how she can stop them from going to war. But she suggests you all make it to the middle of the battlefield and try to stop the charge.
Using a spell to make their voices louder, Sam and Maeve try to dissaude the fighting, and tell the armies about the undead, but nobody listens. But then Aeneas' voice cries out a command, and the revenants begin to pour down into the valley. Maeve says you must try to fight them.
In the face of the undead army, the Romans and Picts finally listen, and the Romans form a tight formation around the Pictish troops. But there are so many revenants, Sam doesn't think you'll be able to stop them. Cerberus appears nearby with a howl, having followed you three back from the afterlife. It gives Maeve an idea, but she must touch Cerberus for it to work.
Just before Cerberus pounces on you, Maeve touches one of his heads, pulling all three of you inside of his mind. You each see out of one of his heads, and can even see your own bodies, still below him. Sam catches the scent of Aeneas, and Maeve says that the only way to destroy a spell is to destroy the spell-caster, so you make haste towards him while still inside of Cerberus.
With the help of your new, massive dog body, you manage to kill Aeneas. But the portal to Tartarus is still open, and if Cerberus could get through, so could many other unspeakable things, so you must rush to the portal in your human bodies to close it.
On the run towards the wall, Maeve says she misjudged both of you. Now she's been inside your minds, she sees your intentions were pure, and that Sam could make a good leader. She thinks together they could restore the land and lead a more peaceful civilisation. It's one last sprint towards the portal, now.
You reach the portal but find its been corrupted, and will only close from the inside. Maeve thinks she'll sacrifice herself, but Orla, her wife, appears from the portal, and reveals she has been returned to the proper Pictish afterlife. After a few tearful words with Maeve, she closes the portal up, with her inside it still. You, Sam and Maeve must now prepare to build a better world.