Counterspell

When a creature casts a spell, you may cast this spell to try a test of Counterspelling against a DC based on the spell to stop that spell, unless the caster is hidden and it is a subtle spell or it is a secret spell.

Roll = d20 + academics + Counterspell bonus

The DC to counter a spell is:

8 + 4 if the caster has mastered the spell + twice the spell’s level

Some casters may have additional bonuses to the DC to counter their spells.
(A spell’s level is normally the value of the hundreds place in its course code.
Spells with direct upgrades are the level of the highest level upgrade applied.
Certain abilities such as Intensify can boost the level of a spell.)

If you pass the test, their spell has no effect (and its cost is not refunded, either).
If you fail to counter a spell, you may try to counter it again by casting Counterspell again, unless you already lost a spell war over that spell.

A caster who has had their spell countered is generally aware that their spell was countered, and by whom.

Identification:
It is easy enough to tell when most spells are being cast, but you will probably want to know what a spell is and what it does before countering it. As a spell is being cast, you may try to quickly identify that spell with a test of Spellcraft, before trying to cast Counterspell.
If you know how to cast the spell, you automatically identify it.
The DC to identify an ordinary spell is 10. More obscure spells may be around DC 15.

Spell Wars:
Whenever anyone tries to Counterspell a Counterspell, it starts a spell war, which has special rules.
A spell war is a battle between two sides,
those trying to stop the original spell,
and those trying to force the original spell through by stopping the opposition’s countermagic.
Any casters that are able to use Counterspell and are in a position to do so may join either side of a spell war.

A spell war is a group contest of skill with a simple objective.
The side that rolls the highest Counterspell test wins.
Or if someone crits on their Counterspell test, whichever side crits the most Counterspell tests wins.

A spell war starts with the two Counterspell tests that started the war.
The side that whoever rolled higher is on is winning the spell war,
and the side that whoever rolled lower is on gets the first turn.
If they tie, the roll with the higher bonus wins, if the bonuses are the same, the Headmaster breaks the tie.

Each side then takes turns trying to beat the other side’s highest Counterspell test until one side fails.
During each side’s turn, whenever they want, each caster on that side of the spell war may cast Counterspell as many times as they want to (and can afford).
A caster may decide to not cast Counterspell during that turn.

Whenever someone beats the opposition’s highest Counterspell test (or crits), their side is now winning the spell war, their roll becomes the roll the other side must now try to beat, and it becomes the other side’s turn.

Criticals in a spell war can only be beaten by other criticals, and always beat the other side (Regardless of if they are higher or lower than the opposing Counterspell test after bonuses are added).

If a side fails to beat the other side’s highest Counterspell test and everyone on that side cannot or chooses not to Counterspell anymore when prompted, the spell war ends and they lose.

When the war ends, the original spell is either cast or countered, whichever the winning side was aiming to achieve.

Finality
Except in unusual circumstances (ex: arriving late against a spell that takes multiple rounds to cast), once a spell war is resolved, no further attempts to Counterspell the spell it was fought over or to Counterspell any Counterspells cast in the spell war are permitted.
(Do remember that everyone who can Counterspell is able to join a spell war, and spell wars do not end until a side gives up or is incapable of continuing).

Time
The magic of a spell war causes time to warp for the participants. A spell war can feel like an epic struggle that lasts for minutes, hours, or, according to legend, eons, but to outside observers it is over within the space of seconds.