These feats are custom to Baldurs Gate: The Sword Coast Chronicles. They were implemented based on their Pencil and Paper incarnations and the work Obsidian had initially done to include them in NWN2.

This is a free feat since it is a base action, any character will receive it for free if elligeble at level 1 of his class!
Type of Feat: General
Prerequisites: None
Specifics: To use a counterspell, you must select an opponent as the target of the counterspell. You may still move at full speed as well as attack and cast spells but any action other than walking will leave you unable to perform the counterspell
If the target of your counterspell tries to cast a spell, a Spellcraft check (DC 15 + the spell's level) is made. If the check succeeds, you correctly identify the opponent’s spell and can attempt to counter it. If the check fails, you can still attempt to dispel but you cannot use the same or an opposite version of the spell. It is possible to cast any spell as a counterspell. By doing so, you are using the spell’s energy to disrupt the casting of the same spell by another person or monster. Counterspelling works even if one spell is divine and the countered spell is arcane. To complete the action, you must then cast the correct spell. As a general rule a spell can always counter itself but some spells are natural opposites such as haste and slow which will also negate each other. If you are able to cast the same spell or a natural counter and you have it prepared (if you prepare spells), you cast it, altering it slightly to create a counterspell effect. If the target is within range, both spells automatically negate each other with no other results. Otherwise a dispel check is made if you have any dispelling spells memorized. Metamagic effects will make dispel and identify checks harder with quickened spells being most difficult to counter. The general rule is DC + 2 for metamagic but DC + 4 for quickened spells.
If there are multiple options to counter a spell with the lowest level spell possible is selected to prevent waste. When dispelling this is not the case, the system will always choose a dispel that has a reasonable chance of success unless it has no other choice.
Use: Selected

Improved Counterspelling
Type of Feat: Spellcasting
Prerequisites: Counterspelling
Required For: High Arcana: Mastery Counterspelling (Archmage)
Specifics: When counterspelling, you may use a spell of the same school that is one or more spell levels higher than the target spell. The lowest level spell possible is selected.
Use: Automatic

Reactive Counterspelling
Type of Feat: Spellcasting
Prerequisites: Improved Counterspell, Improved Initiative
Specifics: Once per round, you can counterspell an opponent's spell even if you have not readied an action to do so. This counterspell action takes the place of your next turn.
Use: Automatic if the mode is on

Epic Counterspelling
Type of Feat: Spellcasting
Prerequisites: Reactive Counterspell, Improved Iniative, 30 ranks in Spellcraft
Specifics: Any number of times per round, you can counterspell an opponent's spell even if you have not readied an action to do so. Such a counterspell doesn't count against your later actions for the round.
Use: Automatic

Feats curtesy of Rasael, testing by the Quality Control Team.