Spell list/Adding new spells
Adding and removing content overview
Perhaps DM has got a great idea for a spell not on the list yet. Maybe one of the players has a character who's supposed to be a clever mage, and the campaign happens to have a long stretch of downtime going on, so they want to spend that time inventing a new spell. In any case, the DM should have final say on the spell's design, ideally with input from players.
There are two different concerns when balancing a new spell: game balance, and setting balance. A spell can have significant effects on setting balance without being very useful for solving problems that the players encounter; although a spell can't really make fights totally lopsided without drastically altering the setting's power dynamics. Take time skip for example: it affects the setting by letting people from the distant past pop up into the world seemingly out of nowhere, but a player character who uses it for such a long stretch of time essentially just removes theirself from the campaign, so in conflicts it's mostly only good for certain escape and sneaking maneuvers. If the goal had been to minimize its setting influence, it could easily have been made Legend tier instead; those escape and sneaking maneuvers aren't quite attainable by any other means, so the spell would still be somewhat useful for those few Legend-tier characters who could cast it. For an opposite example: imagine if bone bolts was far stronger: nearly every character who wanted to be good at fighting would have to be able to cast bone bolts a few times per combat; meaning that every faction would want to equip their army with white gems.
If the spell idea is sufficiently similar to an existing spell, strongly consider adding a Boost to the existing spell instead. The idea with Boosts is that they're a bit easier to learn, because each Boost does not diversify the character's power set as much as an entirely new spell does.
