Hiya!
I don't actually make many threads here, so I'll just get to the point.
I have a species called Viltroids. Basically, humanoids that wear A LOT of clothing. Often dark in colour, and can have one eye, to multiple eyes. Sometimes none at all. They also have horns.
These horns, what I have so far, store their 'magic.' Because of this, removal of the horns significantly weaken them, and even the removal is painful. The horns come in different shapes and sizes. The species can all use magic to an extent; though it really has no effect on anything without proper schooling first. Some Vilts may not even know what magic is depending where they're from.
They rarely use the magic in most cultures but for enhancement or medical purposes.
If anyone could offer some questions to help me flesh out the idea of this magic, you'd be welcomed! As well as ideas, too.
Well, you've got the physiology down. Now there just needs to be a magic system that they can use. Do you have any ideas on what kind of magic they could use? I recommend starting with the Dungeons & Dragons schools of magic and seeing if anything sticks out to you.
does the size of their horns have anything to do with how powerful they are? and are there other living things that can do magic?
Building on @Rach's idea, do the shape of the horns affect the kind of magic? Where do they get their magic from if their horns are removed? Also, does the magic in their horns automatically refill after it is used? If so, who refills it? If not, where must they get it?
can their horns even run out of magic? does it sap their energy to use magic?
Maybe the horns depending on the size, shape and colour the Viltroids could have different abilities so there is some difference and uniqueness to each one? Like if one had ram horns (just off the top of my head I’m not sure what you imagine) that is a orange-red colour the Vilt would have some kind of Fire/lava abilities? And the size could be how strong they are so one with smaller horns are weaker to those with bigger ones?
Or even bette rage horns grow better with age? So older Viltroids have bigger horns because they have more power while younger ones have smaller ones because they are weaker?
If some Viltroids aren't aware of magic, is that partly because of isolation? Are there multiple subcultures of their race with varying levels of magical knowledge? Is there a caste system perhaps, where only the privileged are allowed/know how to use magic (like in medieval times when only nobles and church officials knew how to read)?
I'm also curious about the detail you mentioned, that they wear lots of clothes…what is the reason for this? Do they live in a harsh environment? Is it based of a tradition of modesty? Do their clothes have a more practical or aesthetic design?
Oh, and what determines the number of eyes per individual? Is it genetic? Linked to their magic? Determined by the requirements of the environment they were born in?
It's a very interesting species so far, I wish you luck with developing it further! ^^