You need to find a Realmic Transmutor in Nightingale if you want to activate a Minor card. These differ from Realm Cards, which are used to create different kinds of realms. Minor cards are instead used to change the rules of the current realm, so you can gain an advantage, depending on what you want to do. 

One Minor card might increase your travel speed, while another might make certain enemies drop more loot, but there will often be a negative effect to balance out any benefit you gain. There are 50 Minor cards to discover in Nightingale currently, and I've listed the ones I've discovered below so you can make the most of the Realmic Transmutor in Nightingale.

Nightingale Realmic Transmutor: How to use Minor cards 

The Realmic Transmutor is a stone structure with a globe in the centre. Each realm has one but they're not in a set location, so you might need to hunt around a bit before you locate it. Once you do, you can use the console to activate a Minor Card, which will change the rules of the current realm based on which one you use.

Only one Minor card can be activated at a time, but you can switch them out whenever you wish. Just be aware that you'll lose the card once it's been used, so think carefully before you waste them.

Nightingale Minor cards list 

As mentioned above, Minor cards can give you an advantage in certain situations, so it's good to be aware of which ones are out there. They can be found in chests as a random drop, or you can craft them if you have the recipe.

In some cases, there appears to be more than one type of the same card, which increases its benefit but also adds a negative effect. It's important to be aware of what you're getting into before you activate your card and set off, otherwise, you might be in for a surprise.

Here is a list of the Minor cards I've found, along with what they do:

• Feast: Prolong the effects of a meal, grow hungry more slowly and heal more quickly.
• Tempest: Bring perpetual rains and increase the potency of your magick.
• Might: Increase your strength and carrying capacity at the cost of your tools’ and clothing’s durability.
• Duelist: Increase the damage you deal as well as the damage you receive.
• Maleficiate: Plunge the realm into eternal night, and increase the yield of the Bound’s resources.
• Maleficiate Apogean: Plunge the realm into eternal night, greatly increasing the yield of the Bound’s resources, but dealing consistent damage to you over time.
• Combatant’s Workshop: Craft tools that deal greater damage but offer reduced yield.
• Explorer Eminent: Travel more quickly, extending the benefits of your meals and improving your resistances to most forms of damage while reducing the damage you can deal.
• Explorer: Unconfirmed.
• Angler: Unconfirmed.

The final two cards in the list are ones I've seen but didn't get a chance to catch the description. It could be surmised that the Explorer card would be the same as the Explorer Eminent card just above it, but without the negative effect—reduced damage, in this case. I would also guess that the Angler card increases your fishing yield, but I haven't been able to confirm that.

I'll update this list as more Minor cards are discovered.