Realm Cards allow players to exert control over how they travel through the portal network and connect to new realms. Players craft Realm Cards using materials they find in the environment, with each Realm Card acting as a modifier that determines elements such as environment, weather, resources, creatures and more. Realm Cards can then be combined within a Portal Machine to connect to a new, unique realm. As a gameplay mechanic, Realm Cards take advantage of Nightingale’s sophisticated procedural generation system. 
Realm Cards are the key that unlocks access to new realms, each with environments, creatures, and discoveries to be explored. Using Realm Cards lets you apply agency and choice over what kind of realm you’re going to open. This allows you to connect to one that fits your current needs and desires for adventure. You will need to explore a realm and acquire magical ingredients that allow you to learn recipes to craft new realm cards. 
You can select a limited number of cards to play in combination when you're ready to open a portal. The combination that you play will give you the possible realms within the universe that you are heading to next. Some of the cards will have a major effect on the environment of your destination. Other cards will have more specific impacts, such as weather effects, creature behavior, and plant life.
Cards often do not have a singular impact. They will behave differently in combination with other cards. This will impact the realm in deeper ways, creating an aggregate effect.
Realm Cards define the environment of the realm you create, but using the same set of realm cards again won't generate the exact same realm (no "seed" mechanic).