Eraser is Error and Reboot is not Ink... he's their kid.

In the first multiverse Eraser managed to kill Reboot, but as they were both gods all that's left of Reboot was his Soul which he used as his own. After he destroyed everything the only thing left was the Anti-Void and the original Undertale.

He finally thinking over what he had done and finally realising that he was in love with Reboot and was jealous of all the AU's taking up all of his attention.

Feeling regret he decided to watch over the last au from a distance and after some time fell asleep.

Meanwhile a new being woke up, Ink who was born from the battle remains of Eraser and Reboot, started to create nonstop.

Eraser the destroyer who was asleep started to change as he wasn't meant to have the creators soul, picking up the changes from the soul (e.g. bone colours and use of strings) and forgetting everything but his duty woke up and rampaged seeing Ink and knowing he was not ment to be doing what he was doing.

After a long time Ink and the council find out about Error having The Creators soul in his possession and decided that they would take it and give it to ink as he was soulless so it probably belonged to him.

Seconds after extracting the soul Error changes back to his true form of Eraser with all his memories and original abilities back, and takes back the soul while on another rampage. Eventually Reboot does well...reboot back into existence and everyone finally finds out where Ink came from.

A scene I can see happening:

Reboot: Look at Ink.

"Eraser"

"Ya, what is it boots?"

"We didn't have a kid right?"

"No we were always fighting... why?"

Reboot: Points at Ink.

"He looks exactly like you, if you created instead of destroying."

Eraser: Takes a closer look at Ink and winces.

"Shit... when did that happen and it looks like he got his soulless soullessness from me."

Ink: wide eyed, open mouthed staring.  /(0o0)\