Counterspell has been significantly nerfed in the 2024 D&D rules, perhaps so much so that it is almost not worth assigning a 3rd level spell slot to it. While some degree of "nerfing" may have been called for, I feel as though it has gone too far, so I propose an alternative ruleset for Counterspell, in addition to a different concept borrowed from D&D 2nd Edition, which I'm calling "Mirror Spell"

But, before we start, I introduced a mechanic in our blog on Homebrew which I referred to as 'Secret Spells'... I'll repeat that here for clarity:

Secret Spells

Spellcasters don't announce the spells they are casting before they cast them, so players shouldn't automatically know what spell an enemy is casting. When a creature a player can see or hear begins casting a spell with verbal, somatic or material components, the DM will not automatically announce to the players what spell is being cast.

Players may identify the spell being cast by succeeding an arcana check with a DC equal to 8 + the caster's proficiency bonus + the spell's level (ignoring any upcast, i.e. if a 3rd level spell were being upcast to 5th level, the arcana check treats the spell as level 3). Players who can make a reaction in response to spellcasting (i.e. characters who can use 'Counterspell') can complete this check before they decide to take their action or not.

Mechanically, given only players who can make a reaction in response to spellcasting will benefit from knowing the spell, they should be the only ones allowed to make the arcana check. Other characters may succeed in identifying the spell, but being unable to do anything to stop it means it will make no practical difference whether they learn of the spell before, or after it is cast.

If the spell has verbal components then the player must be able to hear them to make the check. If the spell has somatic and/or material components, then the player must be able to see them to make the check. If the spell has verbal and somatic, or verbal and material (or all three) components and the player can only hear OR see the caster (not both), then they can make the arcana check with disadvantage.

The arcana check is made with advantage (or cancelled disadvantage) if:

• The spell is on the player's class spell list AND the player can cast spells of level of the spell being cast (again, ignoring any upcast) - whether or not is is known or prepared, OR
• The player has seen this same enemy cast the same spell before.

On a successful arcana check, the player learns the spell being cast, and the level at which the spell is being cast. On a failure, they learn nothing.

The player can still choose to take their reaction (i.e. Counterspell) even if they fail to identify the spell being cast, they just won't know the spell they are countering (or otherwise reacting to)

Counterspell

Knowing what spell a caster is casting improves the chances that the Counterspell will work. If the character casting 'Counterspell' has successfully identified the spell, then they impose Disadvantage on the caster's Constitution saving throw to prevent the spell being countered.

Furthermore, for every level spell slot that the Counterspell is being cast above the slot at which the original spell is being cast, the DC for the constitution saving throw increases by 2.

Mirror Spell

Occasionally, a player may determine that the spell being cast is one that they themselves can cast, and have prepared. If this happens, and the player has successfully identified the spell, the player can take a reaction to cast the same spell (at an equal spell slot level or higher than the original spell being cast) but "mirrored", effectively neutralizing the original spell, and the mirror spell.

For the purposes of the Mirror Spell, we will define the 'target' as the caster of the original spell, and the 'caster' as the caster of the mirrored spell.

Requirements

1. The caster has successfully identified the spell being cast (via the secret spells mechanic above).
2. The caster knows the spell being cast, and has it prepared.
3. The spell being cast has a casting time of an action, or a bonus action.
4. The caster has a spell slot available that is equal to or higher than the level of the target's spell (it will be expended by casting the Mirror Spell).
5. The caster has their reaction available.

Resolution

1. The target's original spell, and the caster's mirrored spell collide and possibly negate one another.
2. Both the caster and the target have their spell slots expended (as this is effectively two castings of a spell, offset from one another so as to negate their energies - this is different from Counterspell, which simply snuffs out a spell being cast, meaning the spell slot of the target is not expended)
3. The target's action is spent.
4. The target makes a Constitution saving throw with disadvantage, against the caster's spell save DC. For every spell slot level the mirrored spell is higher than the target's original spell, the constitution saving throw DC is increased by 2.
5. On a successful save, the spell resolves normally and the mirrored spell fails harmlessly,
6. On a failed save, both spells have no effect as their magical effects cancel completely.