The Lizardmen are allowed to bask in their victory over an army of the undead for an evening, while back at Nazarick the Great One Ains Ooal Gown is debriefing Cocytus before his other guardians.
Momonga got what he wanted: Cocytus forms an original thought—that he doesn’t want to exterminate the Lizardmen. But when he can’t come up with a good reason to spare them, Demiurge comes through with an alternative plan: they’ll make the Lizardmen an experiment in rule without fear.
Everything is going according to Ains’ plans, and his guardians think that’s only natural, but privately, Momonga is excited at the prospect of his guardians learning independent thought and gaining experience through defeat…not that he likes defeat.
That much is clear the next day when he shows up with an enormous magically-armored undead army, catching the still celebratory Lizardmen unawares.
Ains himself shows up, and makes quite an entrance, freezing an entire lake, using Gargantua to throw a huge cube of rock into the middle of it, then having his grunts form an undead staircase.
Ains climbs to the top of the rock with his retinue of guardians, proud as you please, and when Zaryusu and his brother meet them in the lake, Demiurge has them prostrate themselves.
Zaryusu knows he and his people are at the mercy of Ains and his forces, and that they don’t have a hope of victory against such power. However, he also presumes the Great One does not intend to utterly exterminate his people, but wants a proper battle to make things interesting.
If that requires a few sacrificial lambs, with him among them, Zaryusu can live with that, even if it distresses the woman he loves. He asks that she put up with his selfishness this once and hold out hope he’ll come back from the battle in one piece. She seems willing to go along with this…IF he impregnates her first. Love: it’s all about give-and-take.