College Casting

* Gaben homebrew WIP. Talk to DM about using this, probably not balanced in the slightest*

You have been trained in the study of group casting: The practise of coming together with other spellcasters to perform greater feats of magic one alone could not achieve.

Over a long rest, you may prepare spells for group casting equal to your intelligence modifier (minimum of 1). You may choose any spell you have prepared. Alternatively, you may instead prepare one spell that is written down in a scroll, or spellbook that you have spent hours equal to the spell's level studying (this spell need not necessarily be one you can prepare).

After this, you and any other casters who have the same spell prepared for group casting may come together to engage a group casting of said spell.

Resolving a group casting is done in several steps. If a group attempts to cast a spell while in initiative, all casters must hold their turns to act simultaneously on the initiative of caster lowest in the initiative order.

Stages:

1. Choose Spell, pick leader, enter formation.

         All casters must agree on a spell to cast, and agree on a caster to lead the group. The casters must then arrange themselves such that every caster is the same distance from the leader. The casting cannot continue until the casters are arrayed in such a way. If during the casting of the spell, a caster is moved out of this formation, the leader must make a concentration saving throw (DC 10) or have the spell fail. If at any point a caster loses concentration during a casting of a spell, the leader must make this save or have the spell fail.

2. Divide into contributors, choose modifier effects.

The casters must organise themselves into units known as "contributors". The number of casters in a unit of "contributors" depends of the level of spell the group is attempting to cast:

3. Pick target level, expend spell slots

       The leader then chooses a level at which to cast the spell. Each contributor group must then pay enough spell slots to cast the spell. If any single caster contributes spell slots than their highest level spell, they must make a manaburn save.

The total slots contributed is a simple sum of the levels of all spell slots contributed to the casting. If any caster contributes a total amount of spell slots greater than the level of their highest spell slot, that caster must make a manaburn save. (See below)

4. Contributors add effects.

Each contributor can add one modifier effect:

Area. A contributor can increase the radius of an area (circle, sphere, cube, cone) effect spell by one quarter. (eg 20ft becomes 25ft, then 30ft, then 35ft with further contributors. 50ft becomes 62ft, then 74ft)

Range. A contributor can add the range of a spell to itself. (eg. 30ft range becomes 60ft, then 90ft)

Creatures Affected. If a spell affects one creature, one contributor may enable the targeting one additional creature. If a spell affects multiple, each contributor increases the amount targeted by half. (eg, 8 becomes 12, then 16, then 20)

Casting Time. A contributor may divide the casting time by half. (eg. 1 hour becomes 30 minutes, then 15, then 7.)

5. Make checks and resolve manaburn.

• One caster per contributor group must make a cohesion check with their spellcasting ability modifier. The DC for this check is 10 + (the spell being cast's level) + number of different classes amongst contributor group). 
• The leader must then make a total cohesion check with their spellcasting ability modifier. The DC for this check is 10 + (5* the number of failed cohesion checks) + (different classes amongst the group).

If the spell being cast is of a level greater than the leader could cast on their own, they must make a manaburn save.

If any contributor fails the save, the caster who attempted the check must make a manaburn save, and distribute the damage amongst their conributor group.

If the leader fails their total cohesion check, they must make a manaburn save and distribute the damage amongst the whole casting party. Casters who are not the leader have vunerability to this damage. If the leader or more than two thirds of the casters are killed by manaburn, the spell fails.

If the leader takes damage from manaburn, they must make a concentration check. If they fail, the casting of the spell fails. 

6. Resolve spell.

Resolve all effect of the spell.

Manaburn:

A manaburn save is a constitution saving throw (DC 15 + spell's level). On a fail, a caster takes (spell's level)d20 force damage. On a success, they take half as much.