Defensive Field

The caster summons a bubble of protective energy that deflects spells. The bubble is a sphere with a radius of twenty feet. The caster may either cast the spell on themselves, in which case it moves with them, or on a space within range they can see, in which case it is centred on that space and is static.

The bubble has a special pool of HP equal to twice the level the spell was cast at (14 at base level). The bubble is invulnerable to all damage and cannot lose HP in conventional ways. Every time a spell is cast from outside the bubble targeting a creature or space inside the bubble's area, the bubble loses a number of HP equal to the level of the spell cast towards it. The caster of Defensive Field must also make a Concentration check with a DC equal to 9 + the spell's level. If concentration is maintained and the bubble still has HP remaining, the spell that targeted a creature or space within the bubble has absolutely no effect on any creatures or spaces within the bubble (though it still affects spaces outside of the bubble if appropriate - the spell is not dispelled, it simply cannot penetrate the bubble). 

Each subsequent turn the caster can use their action to sustain the bubble. If they use their action for anything else, or cast any other spell while the bubble is active, or lose concentration on the spell, the spell drops instantly. As part of the action to sustain the bubble the caster can choose to siphon power into the bubble, sacrificing up to one spell slot of any level to grant the bubble extra HP equal to the spell slot's level. 

If the bubble is reduced to 0 HP, the caster loses concentration, fails to use their action to sustain the field or casts another spell, the defensive field instantly disappears and the spell ends.

Sound and light still pass normally through the bubble, and it provides no cover. Any creature can enter or exit the bubble freely, even enemies, and projectiles can pass through the bubble without restriction. The defensive field only affects spells. Note that even ally's spells cannot enter the bubble and will reduce its HP as if they were hostile. Spells cast from inside the bubble can move through it freely, and casters inside the bubble can target other creatures or spaces within the bubble without their spell being blocked.

Defensive Shield can be dispelled by the dispel magic spell. If any of the bubble's area overlaps with the area of an antimagic field the overlapping area is suppressed and has no effect.

At Higher Levels: When you cast this spell at 8th-level or higher, in addition to the bubble having more HP (see above) you may also increase its radius up to 20 feet per extra spell level (At 8th-level the radius is up to 40 feet, at 9th-level it is up to 60 feet).