This is an example of the process I follow when designing magic items, with the intention of making them memorable, useful, and entertaining to use.
If you just want to check the final result, click here.
I think magic items are one of the easiest items to design in Dungeons & Dragons 5th edition. Of course, the difficulty also depends on the type of magic item you want to create, but in general, these don’t need to be balanced, you just have to be careful on how you award them, and have in mind the level of your campaign. Although a magic item could unbalance your campaign, the truth is that any item, even a well-designed one, can be very decisive. in your game, but there are always several ways to fix those kinds of problems.
Looking for Inspiration
To write a magic item for your homebrew campaign, or to include in an adventure, the inspiration can come from the story itself, from the background of one of the players, something that completes a powerful enemy, the hook to attract players on an adventure, etc.
I usually do it in several ways, one of them is to invent the name of the item first, and then write a story to it and finally create the game statistics. Another is to be inspired by illustrations, then create a story for it, then give it a name and finally define its game statistics. And another process is to combine separate concepts, which at first seem unrelated, imagine the name of the magic item with that, and create a story for it.
Creating game statistics is always the last step for me, because it’s easy to write statistics for magic items in 5th edition for any idea of abilities that we could have.
For this example I started looking for illustrations in the public domain for inspiration and I chose these two images.
The first is that well-known image of the unicorn, the second image is that of an oil lamp that I really liked.
The first two ideas I had were a magic lamp with healing powers, and a horn that would make a unicorn appear when rubbed. They say you should always discard the first ideas, besides, I don’t think rubbing a horn looks good.
The blanket under the lamp gave me the idea of a magic saddle that allows you to ride a unicorn, no matter who you are. But I liked the idea of summoning unicorns better. There is a horn called the Horn of Valhalla that summons barbarians, but I don’t want a blowable horn, I wanted a more convencional magic item. Then I remembered that not only do lamps summon djinns, rings do so too, so let’s do that.
Writing the rules text
The Conjure Celestial spell can summon a unicorn, so a simple Ring of Conjure Celestial would suffice, but I wanted something more unicorn-themed, so I added more unicorn-related abilities to it.
The unicorn has innate spellcasting and resistance to magic, but if we add all that to the ring, it will become very powerful, that is not bad in itself, because the item can be as powerful as we want it to be, but the more powerful it is, it will be useful for fewer campaigns.
A simple solution for this is charges, and since it already has many abilities, I don’t see any reason to make it more complicated, so this is what I came up with.
The final result
Ring of the Unicorn
Ring, very rare (requires attunement)
While wearing this ring you are immune to diseases, you are immune to poison damage, and you can’t be poisoned.
The ring has 7 charges. It regains 1d6 + 1 expended charges daily at dawn.
As an action, you can expend 1 of the ring’s charges to cast one of the following spells: detect evil and good, druidcraft, pass without trace. Or you can expend 2 of the rings charges to cast one of the following spells: calm emotions, dispel evil and good, entangle.
As a reaction, you can expend 3 of the ring’s charges to gain advantage on saving throws against spells and other magical effects for one minute.
As an action, you can expend 7 of the ring’s charges to summon a unicorn, which appears in an unoccupied space that you can see within 90 ft. The unicorn disappears when it drops to 0 hit points or after 1 hour.
The unicorn is friendly to you and your companions for the duration. Roll initiative for the unicorn, which has its own turns. It obeys any verbal commands that you issue to it (no action required by you), as long as they don’t violate its alignment. If you don’t issue any commands to the unicorn, it defends itself from hostile creatures but otherwise takes no actions.
And finally, I made a quick illustration of a ring with a small horn for this magic item.