Battle Caster Feat

You excel at using and maintaining your spells in the heat of battle. You gain the following benefits:

Prerequisite: The ability to cast at least one spell

• Concentrate Checks. You have advantage on Constitution saving throws that you make to maintain your concentration on a spell.
• Somatic Components. You do not need a free hand to perform a somatic component of a spell (such as being to able perform them while holding a weapon).
• Opprotunity Casting. When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

Source: KT