Demiurge responded to the question in a way Ainz never expected.
Shaking.
Shaking like a frightened and wounded child.
"My father." He said, in contrast to the shaking of his body, right down to the tip of his lashing tail, the archdevil 'spoke' with a quiet confidence. About this, he had no doubts. "My father." He said again, a little bit louder than before. "Lord Ulbert is… was… my father."
A part of Ainz was surprised, 'Demiurge… Demiurge having a familial sentiment?!' And yet it also made perfect sense as he came to understand what the archdevil said.
Ainz reached out to Demiurge, 'Tell him now,' part of himself said, one hand out, like a lifeline, but rather than take the demon's hand, he curled his fingers to invite the archdevil to step closer. Ainz opened his mouth behind the mask, ready to speak, but he couldn't.
At least, he could not say that.
"Demiurge," he said, and when the demon stepped closer, Ainz asked him, "What do you remember of Ulbert?"
A shaky smile took over Demiurge's face, "My Lord, may I speak freely?"
Ainz gave a nod while suppressing his anxiety at the question. 'When I was an office worker, that never ended well for the one who asked it.'
"I have spoken with all the Guardians many times. We have a habit now, a routine of meeting to talk together as friends and colleagues. The topic of the Supreme Beings often comes up, and I know one thing for sure. Nobody, not even Albedo, remembers more than I. Their memories are fragmentary, intermittent. Mine are complete, or so I believe they are." Demiurge said, his eyes could not change expression, but the luster of them seemed to brighten.
"I remember the hour of my making, when I opened my eyes for the first time, when Lord Ulbert touched my face and showed me to the rest of you." Demiurge said and brought a hand up to his right cheek. "He asked you all what you thought, and you all teased him a little, asking why he didn't make a 'cute girl' like Mare. I didn't know at the time why they laughed. But I remember Lord Ulbert saying I was perfect the way I was."
Demiurge's throat formed a lump, "I was so proud of myself. I remember when the invasion happened, I fought as hard as I could against the invasion, protecting my master's home until the Supreme Beings could gather, and Lord Ulbert had me healed first. It was my proudest moment, and I learned all too well… that everything beyond the tomb is a danger that must be crushed or it will crush my home."
"And I remember when I began to see him less. He spoke of the chance to attend 'University'. Of course, my Master, my maker, my father… would create a universe to oversee… and the waiting grew longer… and longer… and I began to see him less… and less… and less." Demiurge was incapable of tears, but the way his tail stiffened and drooped by turns spoke volumes.
"I held out, thinking he would take me with him. Wondering why he hadn't called on me to help… was I not made for that purpose? But what son can truly know their father's mind? And of course… then came the last day. The last time I saw my lord, he handed you his weapons, all his things, and disappeared. He looked at me once, up and down like the day my eyes first saw him… but unlike then, he said nothing. Then he said goodbye to you, and was gone."
His head dipped. "My father walked away from me without a word, he did not take me with him, he must not have wanted me with him. How I failed, I never knew-"
Ainz moved without thinking, his hand darted out and snatched that of Demiurge as soon as it dropped away from the cheek where it lingered for too long. "No." Ainz interjected. He vigorously shook his head. "Your father didn't leave you because of that, he left you behind because you couldn't have gone where he was going."
"There-" Ainz paused, he ran through ways to say what he wanted to, "There was no way for you to survive there."
Ainz engulfed the hand of the archdevil with both of his own, looking up from behind the mask, he began to see Demiurge in a very different light. "All of us loved Nazarick… but… the other realm, the place we were in when not in the tomb… it made many demands of us all. Even the hands of Supreme Beings can be forced by necessity."
Demiurge could not meet the eyes of the Lord whose eyes sought his. Ainz spoke in a hushed but firm whisper, as he would to reassure a small boy uncertain of his father's love after a punishment, "That is the truth, I swear it on my name."
"That is- that is a relief, Lord Ainz. But that is a great deal even for me to take in… may I humbly ask some time to process all of that?"
"Of course… why don't you go on and explain… ah… what you have worked out about my plan, tell me more." Ainz suggested, and the archdevil perked up immediately, though he did not try to withdraw his hand from the clasping of his lord.
When it was over, Ainz pulled his own hands away and reached into his dimensional storage. From it he drew a little statue of a many armed person bearing multiple pearl sized gems. Its eyes were shut as if in death or sleep, and had a deep bronze shade to it.
"You mentioned the use of pop monsters, but this will serve you better. Ulbert made it. This is Armageddon Evil, able to summon many demons not of Nazarick, and there is a perfect place to 'hide' it as well." Ainz explained and placed the little statue into the archdevil's open hand.
Demiurge looked down at the object and cradled it with folded fingers as if it were a child in his palm, or at least, like a child that was not meant to be an experiment or food. "I am deeply, deeply honored, My Lord." Demiurge said with a quiet whisper.
"Use it to benefit my Kingdom." Ainz replied using noble command voice number seven. 'A special thanks to Queen Draudillon for that one.'
"I will, My Lord." Demiurge promised with a great wide smile on his face which bared his sharpest teeth against the dim light of the room, causing them to gleam like steel knives in the sun.
"Oh, and one more thing, Demiurge…" Ainz added, recalling the absolute barbarism of the battle between the frogmen and the lizardmen.
"My Lord?" Demiurge asked.
"Chastise the quagoa… but keep it… within reason. They will be my people too, even if their pride must be humbled. Massacre is a waste." Ainz put it in terms the archdevil would understand, and Demiurge nodded along.
'Right, so nothing wasteful… so he's saying that experiments are preferable by far.' Demiurge's pride in his master rose. 'Nothing wasted, everything gained. Such wisdom is truly fit for the leader of the Supreme Beings.' Demiurge thought and put the item away.
"Of course, My Lord. Nothing will be wasted." He replied, and summoned the gate to disappear.
No sooner had Demiurge vanished through the gate than a dreadful urgent pounding began on the door to Ainz' room and Gondo's voice rang out.
"My Lord! My Lord! It's urgent!"