Rampant Spells

A rampant spell is one that has gotten away from the caster, cast in a wild magic area or Arcane storm or a place of overwhelming magical power they draw in too much for their own ability to handle and the spell has become rampant. A rampant spell is a chaotic thing subject to the whim of the DM but generally, such a spell will target everything in its range. The effect or damage depends greatly on the whim of the DM; however, it is good to note that a sorcerer with an untamed Terrorgheist will likely see some form of projected horror en masse.
Spells with a hostile effect such as damage or control elements with a concentration duration are subject to the rampant spell rule. A rampant spell is spawned when a caster rolls a natural 1 on a concentration check, at this point the spell acts on initiative of 20 in time with lair actions under the control of the DM unless harnessed, which it then acts on the wielders turn using their action. On the spell's turn it will break free and require another check to harness at which point it can be siphoned off reducing the current level of the spell, should the spell reach 0 (cantrip) it is no longer present. Any other option allows for the spell to continue to grow more powerful and the spell will increase in level but gains a mutation of the DM's choice, this mutation includes but isn't limited to changing the type of damage, the appearance, granting it movement of 30 feet, increasing its size, lifting objects, and so on. Should a spell reach 9th level the DM immediately rolls a percentile and consults the Rampant Spell Detonation table for the effect.
Trying to harness the spell happens during the spells turn and requires a reaction and a caster check similar to counterspell or dispel magic with a 1d20 + casting modifier against a DC of 10(8 for 5e)+spell level for a rampant spell level 1 through 5, against a rampant spell of sixth level or higher the DC becomes 15+spell level. Any character with the ability to cast spells as part of their class can attempt this, racial spells DON'T count. Allies who are spell casters, not racial spells, can give up their action on their turn to help harness the spell the first ally to do so gives advantage, after that any new ally adds +2 to the check, this stacks, it often takes numerous mages to suppress a rampant spell but powerful individuals can do it on their own. The allies must be willing to aid the one that is trying to harness the spell in what they want to do with it, otherwise, they make their own attempt to gain control. If multiple mages attempt to control the spell then whoever wins the roll gains control and declares their intent, if they wish to siphon the spell it does not grow in power, if anything else it gains a level. Once the spell is under your control you can use it as normal for the round, this includes whatever mutations have occurred, it is an action to siphon the rampant spell or direct it in any other way.

Rampant Spell Detonation

Rampant Spell Detonation

1-33	Explosion
34-53	Wild Magic
54-73	Living Spell
74-94	Arcane Storm
95-100	Planar rift

Explosion

This is rather simple in that the spell and all the excess energy explodes in a brilliant manner. This can be a simple flash of light and fire or have other effects based on the spell that went rampant. If a spell has a damage type it would use that, if not it deals force damage. If it was a flaming sphere for example it would deal with initial fire damage as well as light everything on fire in the radius of the explosion. The only things that do not burn or suffer this damage are things immune to fire in this case and even stone and metal can take damage from it. The radius of these explosions has two parts the initial blast which is 1d20 square radius, this initial blast takes the normal effect of the explosion/spell at 9th level (using the same type of save). The secondary radius is added onto the end of the first extending the range even further but for much less effect, any damage done is minimum and in the case of the flaming sphere example the fire is mundane so only flammable objects will be set on fire.

Wild Magic

The easiest way to use this is to use the chart with the Wild magic sorcerer if you are using 5e, or any other wild magic table you choose to use. Anytime a spell is cast from anyone withing 1-mile radius from the epicenter the roll is made on the chart. This includes enchanted items that activated, such as wands continuously enchanted items simply cease to function while within the radius. The effect lasts for 1 day and the DM has the final say on how the wild magic takes effect.

Living spell

The spell becomes a living spell.

Arcane Storms

While all of the ramifications of a rampant spell detonating are bad, this is one of the most feared ones as while the other effects are largely localized an Arcane Storm will form over the epicenter but can extend very far and will travel like a storm system.

Alternate methods of dealing with a rampant spell Dispel magic works as normal, use the spells current level as the DC (The ones provided above), if it succeeds the spell is removed. You may allow knowledge checks, generally arcana but arguments can be made for religion and nature as well to learn other methods, such as summoning an arcane elemental which will consume the spell. Or other creatures or beings that can deal with such energies.

Additional bonuses

A tamed or above Terrorgheist can aid the bonded ally to add their proficiency bonus to the attempt, a hostile one will allow some aid in the form of advantage as all Terrorgheists -hate- rampant spells with such a passion it overwrites anything else, if the caster tries to run from such an event, more so if they caused it they will have to make a wisdom save or be compelled to deal with the problem. The DC is up to the DM but it should be difficult.
Other powerful forces of magic can grant similar bonuses, servants of gods of magic should be likewise compelled and benefited by these powers to deal with Rampant spells, often for reasons that seem to go beyond the immediate threat.