Sensory deprivation

Sensory deprivation is the ability to make others unable to utilize their senses: sight, taste, smell, touch, and hearing.

Alec (Twilight)
Like his sister Jane, Alec possesses a powerful gift. It is because of these gifts that they are highly respected and feared throughout the Volturi and the rest of the world of vampires. Alec's gift is to numb the senses of any individual he focuses on. The manifestation of both gifts boils down to their attempted execution at the stake: whilst burning alive, Jane focused on the pain and anger the humans caused her, thus manifested her ability of creating the illusion of burning pain in any one individual she focuses on. Conversely, while being burned, Alec focused on escaping the pain, and his ability manifested into cutting off the senses of any human or vampire.

Alec can selectively or completely block the senses of several people at the same time, which allows the Volturi to easily execute their opponents. When he uses this ability, it manifests into the form of a shimmering hazy mist that slowly moves toward his target, and it cannot be affected by physical materials, such as wind or gravity. Unlike Jane, Alec can affect multiple targets at a time, therefore making him more dangerous than she is. For this reason, he is the Volturi's strongest weapon in combat. However, while Jane's power works almost immediately, Alec's power requires time to take effect. The Volturi sometimes have Alec anesthetize a criminal before he is executed to avoid complication if that criminal had pleased the Volturi in any way.

Since their gifts are long range, he and his sister fight from the sidelines under the protection of bodyguards. His main purpose is to paralyze the senses of the guilty party so that cannot flee or defend themselves during deliberation or execution. For example, when Aro, Marcus, and Caius are counseling, Jane and Alec try to use their powers to stun the Cullens and their witnesses. Upon hitting Bella's shield, the mist went around it, making it appear like a giant bubble. Alec maintained the mist for the entire confrontation until Aro decided to retreat.

Because his power had never been thwarted before, there was no need to train him in combat. Therefore, his battle techniques aren't as efficient as the rest of the guard. If he is unprotected and powerless in a battlefield, he is likely to become an easy target. Bella speculated as much when contemplating a fight with him, although Edward feared that Alec may have attained some skill in his time with the Volturi.