He touches the cardstock. 

"Paper masks are light and flimsy. If worn too long or too often, for more than a few hours a day or a few days a month, they may begin to lose their power. However, out of all the masks, they are the easiest to learn how to safely repair; and damage to them does not harm the bearer at all."

He touches the iron ingots. 

"Metal masks are heavy and strong. It is almost impossible to damage one, but wearing them for long periods is uncomfortable and draining, both physically and spiritually. And if something does damage the mask, the harm to the bearer will be significant."

He touches the leather scraps. 

"Leather masks become part of the bearer's body. Once put on, they will meld with the bearer's flesh and transform it to match the mask's features. They cannot be separated except by killing the bearer, which destroys the mask. Damage to a melded mask is damage to your body; it hurts as much as you would expect, but has no effect on the mask's magic."

Innnnteresting. 

He reaches out to touch the empty space of eddying mist. 

"Soul masks are not only bonded to the bearer's soul, they are directly fused with it. Even death will not separate them. You can summon and dismiss your soul mask at will, and it is immaterial and impervious to physical damage; but magic can harm it while it is manifested, and injuries to the soul are grave indeed."

For reincarnation reasons he's probably going to go with a soul mask. But...

He draws his hand through the air, pulling carbon out of the carbon dioxide. Soon, he has a little lump of diamond in his hand. He holds it out. 

The mist swirls uncertainly around him.

"A mask of diamond..." the voice says slowly. "It could be done. I have never seen it done. I would not have the tools, or the knowledge of how to work the material. At a guess, I think it would be most similar to metal in its nature and effects, but I cannot be sure without seeing the mask made and worn. It might kill you; it might make you immortal. The only way to know is to try."

He nods, and puts the piece of diamond down. 

"To begin with, I choose soul," he says. 

"You can choose to stop working and start over at any time before you bond to the mask," the Assistant assures him. "If you wish to switch to a different material later, it can be done."

A swirl of mist clears away all the other materials, diamond included; then tools appear, of strange form and mysterious function. Some look like carving or leatherworking tools; there's also an eyedropper, a pair of excessively tiny tweezers, a pale jade comb, and, strangest of all, a pair of spectacles with solid silver lenses gleaming in the moonlight.

"Please put on the soulsight spectacles so you can see your work," the Assistant instructs.

"The next decision to be made is the form of your mask. All forms of mask can hold one elemental Rune and two Decorations, each with its own contribution to the mask's power; a Crown-shaped mask, having no face covering at all, provides only those. An upper-face mask will additionally have Eyes, and a lower-face mask will have a Nose and Mouth; a full-face mask will have all three, and a full-head mask can bear one more Decoration. Each mask also requires two Complications to balance its power, and a full-head mask requires two more than that."

He puts on the spectacles. "I'll try a full-face mask, then," he says. 

When the spectacles are off, the lenses look fully opaque. When they're on, though, he can still see the world through them—very dim and blurry, but those are his hands, that's the tree stump, that's the comb...

...and, piled up on the tree stump, right where the mist has been getting so excited, a wispy little heap of glowing something-or-other. It shines brightly through the silver lenses, the only clear and colourful thing in an otherwise washed-out world.

"Next, the style. There are many possibilities, but the five most common are Plain, Ornate, Demon, Skull, and Beast. A plain mask hides itself from view and is easily overlooked, but cannot bear any Decorations; an ornate mask needs an extra Decoration, and draws attention so compellingly that even magical invisibility cannot hide it. A demon mask is styled in the form of a human face with exaggerated features; it grants the bearer enhancements to strength, health, and endurance, and encourages their body to grow taller and more muscular. A skull mask is styled in the form of a skull; it grants the wearer the ability to summon and communicate with the spirits of the dead. A beast mask is shaped like the face or head of an animal; it grants the wearer the ability to transform into that animal at will."

"Will a soul mask be visible to someone not be wearing a set of these spectacles?" he asks, tapping the frame. 

"Raw soulstuff is invisible to the untrained and unaided eye, but as you craft your mask it will become more and more visible; once bonded and worn, a soul mask is usually only a little translucent."

"So whatever the mask looks like, that's what I'm going to look like, permanently?" 

He's slightly perturbed by this concept, and irritated with himself for it. Vanity is a trait he's never had a need to purge from himself, but if it gets in his way now...

"Since it is a soul mask, you can manifest and dismiss it at will. You will only look the way the mask looks while you are wearing it. Or, if you craft a Plain mask, you can make it subtle enough that only those who look very closely indeed will notice it, and everyone else will see only your face."

If he can turn it off at will that's fine, then. On to actually important questions: "What do Decorations do?"

"I usually offer a choice of the ten Decorations I am most familiar with; each one has its own unique magic. For example, Scales grant immunity to mundane physical injury, and Gems transform the bearer's body into their ideal form while they wear the mask. There are other Decorations known to me besides the ten in my usual list, but I cannot craft them so readily, and their magic might be more limited or less reliable."

"What does it mean that an Ornate mask 'needs an extra decoration;' is it possible to purchase additional ones in some way?"

"The Ornate mask can bear an extra Decoration, like the full-head mask; but it is possible to craft a full-head mask with only two Decorations if for some reason you want to do that, whereas an Ornate mask must have three, and an Ornate full-head mask must have four. One of the available Complications also grants extra Decorations, but at the cost of making all Decorations slightly less reliable."

"Understood. What exactly does 'the ability to summon and communicate with the spirits of the dead' mean?"

"When someone dies, their spirit may linger, or it may pass on. Lingering spirits cannot be seen or heard directly except using spirit magic, or tools made with spirit magic. The bearer of a skull mask, while wearing their mask, can see and hear those spirits, and can call out to a recently passed spirit to summon them back across the divide for a brief time."

"So it will do little in cases where someone has been dead for a very long time, or in worlds where lingering spirits are less of a phenomenon."