Establishing the contract was fairly easy and the process slightly fascinated Tony. Not the visual part of it, of course. But the unknown energies he could feel mingling and intertwining, energies that were definitely not 'magical', as they left the parchment when the signatures were confirmed before they merged into both him and Natasha... it was an entirely new experience and an entirely new kind of energy for Tony.
He didn't feel any different though. No matter how he looked for it, he could find what changed and that was a bit scary. He was glad the contract needed full consent and understanding because without that, the possibilities to abuse this were enormous.
Naturally, this led to very long talks where Tony tried to get every information Tao knew about contracts, oaths, and similarly binding magics.
Turns out, much to Tony's relief, there was no chance of someone swearing a magical oath without being first fully aware of what he is doing. It was less in the spoken words and more in the intent of the user mixing with his magic in a specific way. Something like accidentally swearing an oath could not happen.
Well, contracts could contain hidden clauses, and not every one of them demanded the recipients to understand the contents so Tao quickly cautioned Tony from signing anything without first fully reading it. Especially when it came to the mystical world.
Tao knew how reckless Tony could be. Especially when it came to something he wasn't interested in like paperwork.
This wasn't a problem for non-magical people. If it was, the mundane world would have been conquered by the various hidden magical societies a long time ago through this method.
Alas, signing a magical contract required the signee a certain amount of magic. An amount that non-magical people simply did not possess.
That's why Mephisto and his ilk usually opted to make use of soul contracts. Even a normal person could sign them as there was no need for magic and they could use these contracts as a leeway of a sort from the restrictions placed on them. It allowed them to even give powers to those acting as their agents which more often than not made Tao's job to clean up after these reckless flaming skeletons of Mephisto that much more headache-inducing.
She was only glad that other Demon Lords did not make a hobby out of giving barely controllable powers to normal humans for kicks and giggles in exchange for their soul.
Them using a variant of soul contracts was an upside in itself though. No human could be fully tricked into signing such a contract. They may not be aware of what exactly would be happening to them, depending on what kind of variant the Demon Lord was using, but they knew they were signing away their soul.
That's why Tao never intervened in this.
The Demon Lords were not as big of a problem for Earth right now anyway...
Tony was also a bit surprised when Tao revealed they made a new discovery while trying to put together the parchments capable of being used for the soul contract.
Apparently, Natasha had a massive aptitude for soul magic and energies.
The creation of this parchment should normally take a few weeks to be taught, yet, due to the mix of her determination and aptitude, Natasha learned and created one in under a day.
It was just a pity she did not have the same aptitude for other energies and magics...
Nevertheless, this gave Natasha a clear direction in which her studies would now progress. Becoming passable at everything was a must but it would have been foolish not to prioritize one's natural talents.
Tony instantly asked what are his supposed natural talents when it came to the Mystic Arts and if there was a way to find out... but Tao quickly rebuked him with a stern 'NO' for some reason.
Heh, the old hag probably did not have such a way and just tried to spare herself some embarrassment. Tony was sure of it.
...
Tony groggily woke up from the sweet clutches of his sleep and instantly felt something snuggled against his chest. Looking down, he saw a mess of red hair clutching at his shirt while peacefully sleeping on his chest with a small satisfied smile...
The first thought in Tony's muddled head was wondering if he did it again and fell back to his old habits but it didn't last longer than a second as his brain rebooted and he realized it was just Natasha.
A second after that, he remembered they came home early in the morning after establishing the soul contract between them and sat on the couch next to each other utterly exhausted...
And then nothing.
Considering they were both clothed and at the time in no mood for any other nightly activity other than sleep, Tony doubted anything happened between them.
He didn't know if he should be relieved or disappointed because of that.
It seemed his lower brains had it all figured out, though. If the way it stood on attention was any indication.
'Down boy.' Tony scolded himself, chuckling inwardly at the situation he found himself in while his hand started to involuntarily caress Natasha's hair.
Tony thought he was gentle enough to not wake her up but it seemed super spies were trained to easily get up at the slightest disturbance. Who would have known...
Natasha obviously stirred and instantly opened her eyes, about to push herself up... only to relax a second later when she realized where she was and her lips stretched into a small indulgent smile as she snuggled again into Tony's chest, sighing in content.
"Go back to sleep, Tony." She mumbled and closed her eyes, not protesting one bit to his hand on her head.
Needless to say, Tony felt flabbergasted that he was now supposed to be her hug pillow for who knows how long. He was wide awake and wouldn't be falling back to sleep so easily!
'Great. I can't even get away because she is laying right on top of me.' He mentally sighed. 'This relationship thing already sucks...'