Battle Caster A5E

Type: General
Prerequisite: Requires the ability to cast at least one spell of 1st-level or higher

You're comfortable casting, even in the chaos of battle.

You gain a 1d6 expertise die on concentration checks to maintain spells you have cast.
While wielding weapons and shields, you may cast spells with a seen component.
Instead of making an opportunity attack with a weapon, you may use your reaction to cast a spell with a casting time of 1 action. The spell must be one that only targets that creature.

Source: Adventurer's Guide (EN Publishing) · License: Creative Commons Attribution 4.0 / OPEN GAME LICENSE Version 1.0a details