I think it's very important to this tone that Ewing never bothers to explain what Xemnu IS. No flashback to that first Journey into Mystery issue, no rundown of his origin story--and Ewing loves those moments! Xemnu just EXISTS. Maybe he always has.
You remember Xemnu.
I controversially think the Xemnu arc of Immortal is weak--too long is spent on the kaiju lead-in, too little on Xemnu's Magic Planet--but I still think Xemnu is special. He feels like a bizarre, prototypical lifeform, not alien or villain but not NOT those things. It's creepy!