Imagine Momonga- in his early stages, when he was still human- missing all the little things in life. At one point, he mentions Peggle.
"What's that, Momonga-sama?" Aura asks.
"Well, it's a 2D game where you shoot a ball at pegs in order to clear them. You only need to clear the orange pegs, not the blue. Oh, and there are green and purple pegs as well. The purple is a bonus peg that changes position every shot. The green gives you a boost of some kind. I liked Zen Shot, myself, because it-" He notices them staring and clears his throat. "Excuse me. I did not mean to prattle on."
Shalltear poses dramatically. "Nothing you could say would ever bore me, Momonga-sama!" She throws herself at him, which annoys Aura.
"Instead of focusing on your own pleasure, Shalltear, you should have focused on Momonga-sama's words." They exchange a glare, but Aura continues before Shalltear can interrupt. "Momonga-sama," she bows deeply to him, "please tell me more of this game."
Momonga is puzzled, but sees no reason to refuse. Shalltear is just happy hanging off him- she is paying attention, thankyouverymuch, and For The Record, she is trying to make sure the Glorious Momonga-sama has every opportunity to enjoy himself. Whatever form that may take.
Aura has listened to Momonga-sama's explanation very carefully. A plan has formed in her mind. She's made a list of all the power-ups and features, puzzled over the 2D aspects, and worried about the size and scale of the project. Shalltear is still in the room, but no longer hanging off of Momonga-sama. She's been giving Aura a look that asks, What are you up to? Aura forces herself to admit that Shalltear is smarter than people give her credit for.
Momonga has taken the time to enthusiastically explain every detail. He's still not sure why they're doing this, but damned if he isn't excited to talk game design again. That was one thing he really missed about this world. No technology. A medieval, tech-barren wasteland. Even the smartest people on the planet wouldn't be able to understand the concept of video games.
But Aura understands bowling. And what is bowling but knocking down pegs? Momonga feels like jumping for joy. She really seems interested in it- maybe she wants to make her own game? Either way and anyway, Momonga is happy to play around. By the end of their talk, he doesn't feel quite so lonely anymore.
Aura spends a lot of time trying to figure out how to make a real life version of Peggle. The worst part is the 2D. A real ball could bounce out of the playing field, and a glass barrier would cause its own problems. Controlling the ball with magic wouldn't give the randomness Momonga-sama said he loved. (Plus...whoever controlled the ball would probably just cheat and make sure he won. Which isn't what he wants.) In order to recreate Peggle perfectly, the way Momonga-sama remembers it, she's going to need a lot of help.
Aura asks around and is told that the insect people of Nazarick will be the most helpful. Bugs are really, really, gross, but if it's for Momonga-sama, she'll do anything. Steeling herself, Aura asks if there are any who can curl into a ball.
Hurdle 1 cleared!
Shalltear notices when Aura clears a field near the Great Tomb. This time Aura's going to answer her. She walks over.
Aura is busy building a giant slope. There's a cannon at the top, and a platform overlooking the whole thing. It doesn't seem to be a battlefield.
Shalltear shouts. "Aura Bella Fiore!" She smirks when the accused startles, turns around, and snarls. "Just what do you think it is you're doing, my dear?"
Aura wants to strangle her but refrains. Shalltear could be very useful. She explains the plan and adds, "Momonga-sama really seemed to enjoy talking about this game, didn't he? I want him to feel at home."
Shalltear's stern but impassive expression breaks into a scheming one. Aura knows it isn't directed at her- Shalltear may sabotage Aura, but she'd never sabotage Momonga-sama's present. Aura waits. "Mmm...yes..." Shalltear finally says, "this could work. And with my summoning abilities...hmm...yes..."
It's the big day. Everything is finished. But will Momonga-sama (or rather Ainz-sama) like it now? He's changed so much over the last few weeks. Aura's nervous.
Shalltear's job is to bring Ainz-sama here. She gets to distract herself with his awesome presence. She doesn't have to stand here, waiting.
"Tch!" Aura says grumpily, "she's probably taking her sweet time, too." Aura wants to continue- it feels good to rant about Shalltear's faults- but then she notices a deathly aura. "Ainz-sama!" She cries. Aura runs over, lest she keep Nazarick's Supreme Leader waiting.
Aura shows him everything. She demonstrates the cannon, the various powers to choose from- excluding that awful Claude he expressed displeasure for- and the bucket. "It's not perfect," she mumbles, "but it's what I could do with what I had to work with." Aura shifts from side to side.
Ainz looks over all his wonderful guardians have done for him. The faith to his description is astounding. The ball bounces as if it were 2 dimensional. The pegs look like pegs. The bucket looks just as unforgiving as it always did. And there is even a transition sequence to Extreme Fever! This must have been a very difficult task. A long silence passes as Ainz is overcome by emotion.
"Ainz-sama?" Aura's voice is small.
He raises his arms. "Wonderful! Simply wonderful!" Ainz picks up both Aura and Shalltear, hugging them tightly. "This is the greatest gift you could have given me. No, just to know that you cared to create this. Thank you, Aura. Thank you, Shalltear."
This time, Shalltear doesn't go for the dramatic response. She and Aura just hug back and smile.