Idk, I like to think about concept of like, an heir to Angbands throne.
Example: what if during the war of Wrath, Melkor decides that he might need a plan B, just in case.
So one day he calls Mairon over to help on a project that required "superior craftmanship"
Marion is all "dear god what abomination are you making today" but helps anyway, not knowing what's up
After a little while (Mairon still doesn't know what's going on) theres this flash of light and pop! A tiny thing appears on the anvil or whatever they were working on
Melkor is absolutely stoked because after all this time, he tried to make something of his own instead of corrupting things around him and it actually worked
It takes Mairon a few seconds to comprehend what just happened lol
The kid inherits both of their powers/abilities but it takes a while for them to get a hold of it
The orc captains are scared of the kid because they'll be getting yelled at by the dark lord when this tiny thing walks in, babbles a bunch of nonsense while Melkor is fawning over his perfect little creation. The captains are so confused but they soon learn not to interrupt or ask what hes doing (or who the kid is shhhh it's a secret), or else bonk! Hammer to the head.
Mairon is unimpressed with the kid's lack of speech, mobility and coherent thought, but ends up spending a lot of time with them anyway because if you dont keep your eye on a magic baby/toddler with the ability to shapeshift they might just turn into a key or smth for a couple days and you spend an ungodly amount of time looking for someone who's in your pocket
Melkor insists that they wear full armor for protection, and it's very easy to tell when they're bumbling down the hall when you hear a bunch of bitty *clank clonk* noises of their little boots
It's not easy being raised in the middle of a war but what are ya gonna do
Then when Mairon leaves at the end of the war of Wrath, Melkor has him take the kid with him (I mean, he couldn't not leave the kid with some sort of unresolved childhood trauma, he's the king of daddy issues after all)
Thus, Mairon has to raise them mostly on his own. It gets difficult when he's pulling his schemes on Numenor and all but who's gonna notice another (shapeshifted) teenaged apprentice murking about who showed up suspiciously around the same time as this other new guy?
They get back to living it up again once they're in Mordor, though they have their fair share of household chores
They're not told their name until the third age just before the Ring enters Mordor. This was done so that they wouldn't commit any inherently evil acts until after Mairon was gone, so that if Mairon was brought to trial, they couldn't accuse the kid of anything. Mandos can't convict/imprison one who hasn't done anything wrong. (And even if the kid did do something warranting the wrath of the Valar how are you going to convict nobody of something?)
Like could you imagine, the Vala in the fourth age, having finally rid Eä of both Morgoth and Sauron, when they get word of little Timmy who is committing mass arson and wreaking havoc across middle earth
Idk these are just my Thoughts
Bonus: the kid eventually breaks into Valinor, rescues Melkor from the void and steals whatevers left of Mairon