Create Lesser Mythal

In ages past, when the world was young and the laws of magic less restrictive, ancient high mages developed the first of the mythals: permanent areas were the laws of magic and physics were altered, tailored to whatever those mages of old desired.  Though the power to create true mythals has long gone from the world and is out of the grasp of even the most capable mage, diminished replicas of those mighty magics are still possible.  But even diminished as they are, they boast great power, and a steep price.

You create a persistent magical field, called a mythal, with a 100-foot radius centered on the point you can see. When you cast this spell, you can designate up to nine other creatures within 100 feet of you to assist you in creating the mythal. To assist, a creature must use its reaction when you cast this spell to expend a 9th level spell. An expended spell doesn't come into effect, and is instead consumed to fuel the mythal's creation. For each 9th level spell consumed by the mythal, you can increase the radius of the mythal by 100 feet.

Part of the cost of casting this spell is life force. When you cast it, you immediately lose 300 hit points, which can only be reduced by dividing the loss among willing creatures within 100 feet of you. A creature reduced to 0 hit points by this effect dies and can’t be revived by any means while the mythal remains. Neither the spell nor its effects fail as the result of the caster or an assistant dying to this effect, unless the death prevents the full payment of 300 hit point cost.

By casting this spell, you create the framework for the mythal, providing the means for persistent magical effects, but none of the effects themselves. It is up to the creatures assisting you to determine the effects of the mythal.

As part of the reaction used to contribute a spell to fuel the mythal, each assisting creature can create one of the following effects:

• Prevalent Spell. Choose a spell of 5th level or lower that you can cast that can target you, doesn't require an attack roll or saving throw, and has a duration other than "Instantaneous". All creatures are affected by the chosen spell (using your spellcasting ability) while within the mythal's area. The spell only ends when the creature leaves the mythal.
• Arointed Spell. Choose a spell of 9th level or lower that you can cast. The chosen spell has no effect inside the mythal.
• Vanguard Spell. Choose a spell of 3rd level or lower that you can cast. Creatures inside the mythal can cast the chosen spell at will, without expending a spell slot or material components. Creatures with the Spellcasting or Pact Magic feature or an innate spellcasting trait use their spellcasting ability for the spell; otherwise, a creature uses the same ability score you use as your spellcasting ability.
• Prevalent Power. You may choose another effect beyond the scope of those presented above.

In the case of a Prevalent Power effect, the creature states its desired effect to the DM as precisely as possible. The DM has great latitude in ruling what occurs in such an instance. While no effect will outright fail, the effect the caster desires might only be partly achieved, or might have some unforeseen consequence. For example, warding a mythal so that no undead can enter could also cause spells of resurrection fail inside it, as the soul of the resurrectee may be treated as a specter or ghost by the mythal, making it unable to enter the mythal to return to their body.

Once a mythal has been created, it can’t be dispelled by dispel magic, and its effects are only temporarily suppressed in the area of an antimagic field. A mythal can be altered by casting this spell upon the area of an existing mythal. In this case, each assisting creature can change or remove one effect from the existing mythal, but can't add any new ones.

Mythals can't overlap.  If a new one would overlap with an old one, the new casting changes to become a modification of the old mythal as described above.

If the mythal is created with no effects, or its last effect is removed, it instantly collapses and is destroyed.