He's smiling on his way out; he hasn't had a meaningful increase in power in centuries. Promise is the best minion.
And thinks about what things are "minor" and "like that". She is allowed to sit down; is flying like sitting? (Yes, at least short distances.) She can do fairy lights; this allows a wide variety of sorcery. She can speak freely and she still remembers her own name, so if anyone who might be willing to help her comes along and seems like the best chance, she can pull the exact same trick to get away from Alendi that she did to get away from Yellow. They would, however, have to help of their own will at least to start: speaking freely does not let her enforce commands. She could speak them (and even with Alendi's real name forgotten, they might startle him if she timed it right) but they will have no power behind them. Enforcing commands is probably not "minor" or "like that" even without whatever caveat she skipped.
Think, think. (Work, work.)
Alendi is back soon, carrying a tray. "You can use the tin for taste if you want to. Either direction."
"The project on collecting fey food is pretty successful. How much do you think we need?"
"I don't know exactly. It depends on the population and delivery mechanism and whether you want me to plant and get more food from any of it. If you're not going to hand-offer me all the water I drink, too, I'll need to purify it by sorcery to prevent the possibility that there's some mortal something in it."
He can't think of any way it might be, but better safe than sorry.
She reaches for the pitcher and holds it in her lap to do magic to it. And eats from his hand.
He feeds her, thinking about whether it'd be worth killing a Mistborn and a Feruchemist to give Promise the ability to not need food.
"I can start spiking water sources, but it depends how diluted it can get and still work."
"Food vassalizations can be - fuzzy. There isn't an exact proportion that will definitely work. It's a complex interaction between things like my level of claim to the food and how quickly I try to use it."
"Maybe we should have delayed the collection until you could be there to be in charge of it. Well, we can work with what we've got."
"Nobody but me is going to walk in on you here. I'll stay away for the next...few hours? I'm not immediately in a rush, so let me know if you need longer."
"I might not be done with this chunk for as many as seven or eight hours."
If she's going to be in here for long, there's another unpleasant consideration.
"If fairy sorcerers still need to use garderobes, the one attached to the room you used last night will be safest. You may, making sure nobody sees you or otherwise finds out you exist, leave for short amounts of time to use the toilet. Since I didn't think to install an outhose in the hidden entrance to the prison of the god of destruction."
"You can see the drawings after I've done them, if you want. I can turn invisible, but I still contain spikes," she points out. "If anyone who can see those is around I can't prevent them from noticing me."
"Metal that's partly inside someone's body is usually impossible to sense with Allomancy; it's just me and the Inquisitors who can do it, and they wouldn't know those spikes are attached to a person. They rarely have reason to be in the palace anyway. It shouldn't be a problem."
He leaves, closing the door but not sealing it with Allomancy.
And with her spare attention she makes little fairy lights.
He then starts stockpiling his metalminds even further. Especially atium; if he gets stranded in Fairyland it'd be nice to have a life expectancy measured in millennia rather than months.
The next time Promise sees him he's bringing food and water.
"Anything unexpected?" he asks when he enters.
"No," she says absently, and she puts the rock down to attend to the food and water. She purifies the water again.
He offers her some of the food.