One Line of This Spell Makes These Ideas Difficult
There is an essential phrase you (originally) left out of your description of the Shape Water cantrip (Elemental Evil Player's Companion, p. 21, bold added).
- You change the water’s color or opacity. The water must be changed in the same way throughout. This change lasts for 1 hour.
All of your ideas seem to involve selectively changing part of the water to one color, and other parts to another color (e.g. changing the water on the legal document to spell out different words in some places, changing the color to be white in some parts of the die and black on others, changing the water to be white on some parts of the card and red on others to represent an ace, etc.). But since "the water must be changed in the same way throughout," this would most likely be impossible.
It might be possible to write a specific message in water on a paper or card, and then turn all that water black and opaque (or clear) as you desired. But you couldn't alter the message to fit your needs. You could argue that you can "cause the water to form into simple shapes and animate" and thus rewrite the messages or cards as you see fit. But "simple shapes" tends to mean polygons (i.e. closed shapes like triangles and squares), and usually would rule out more intricate shapes like writing.
Finally, it's quite debatable whether a wet card would be considered "an area of water" (or whether a wet document would be anything other than suspicious). You've suggested (in a comment) that the card would not be wet, but covered in "a light layer of colored frost." Note, however, that the spell does not give you the ability to melt ice, but only to freeze water. And ice cannot be moved or altered in appearance or shape by this spell: only water.
The first option - if the target makes a save, your reaction has already been used.
I agree that the sentence by itself is ambiguous. However, from my experience, any feature in 5e that forces a save and requires some kind of action consumes the action regardless of the result of the save. Therefore, I believe the same thing is intended here.
Notably, there are some features that you can choose to use after you know if you succeed, but afaik they all depend on you hitting with an attack.
Regardless, think of the Halo of Spores in-universe: you're using your reaction to blast the opponent with your spores, therefore your reaction is used regardless of the save. You don't ask your opponent "Hey, would you like to get blasted with spores? Yes? Well, here you go!", using your reaction only if he agrees/fails to defy you ;)
Best Answer
Those are Guild symbols
Those symbols are the symbols of each of the ten guilds on the world Ravnica. Each of these guilds has their own theme and core beliefs. A character can choose a guild based on the character's pre-existing beliefs and values, or a character's values can be shaped by the guild they have joined.
In the card game Magic: The Gathering, each of the ten guilds brings its own unique style and mechanics into the game. If Ravnica is used as a D&D setting, then that translates into unique guild mechanics, such as the Golgari Spore Druid, whose spores can damage enemies just for standing next to them, or a Dimir wizard, who gains access to unique spells that can steal memories straight out of a target's mind and send them somewhere else.
While many might prefer to choose a Guild that fits a character, that die allows the player to choose a Ravnican Guild at random instead. Just roll it, and whichever symbol is shown is the character's Guild. It should also be very helpful for generating random NPCs, when the DM either wants a random distribution or doesn't want to think about the decision.
Should you use this die to choose your Guild, be warned - it is not merely a cosmetic choice. Apart from flavor and unique mechanics, the character will be heavily shaped by their Guild, and any character will have a number of Guilds where they simply do not belong. A Lawful Paladin, for example, would not do well in the lawless torture/entertainment scene that is Rakdos, and a Chaotic-Anything character would swiftly be incarcerated and thrown out of the Azorius guild. I recommend only making the choice in this way if you are absolutely fine with completely changing your character, or if you haven't defined your character yet.