Gregory of Nyssa

Certain evil spirits imitating the heavenly hosts and the legions of Angels say that they are legions. As also their prince says that he will exalt his throne above the stars that he may be like to the Most High. (Isaiah 14:13.)

Catena Aurea by Aquinas ↗

Bede

But Jesus asked him, saying: What is your name? And he said: Legion, because many demons have entered into him. He does not inquire the name as if ignorant, but so that the confessed fury endured in front of witnesses, the power of the healer might more gratiously shine forth. …

On the Gospel of Luke ↗

Bonaventure

Fourth, as regards the company of multitude in the demon responding to the Lord's interrogation, it is added: And Jesus asked him, saying: What is your name? For it belongs to him to interrogate, to whom it also belongs to judge: Wisdom 6: "The Most High will examine your works and will search out your thoughts"; and in chapter 11: "These, questioning them as a stern king, you condemned"; because, according to that of Wisdom 1, "there will be an interrogation into the thoughts of the ungodly." He was asking about the name, not …

Commentary on Luke, Chapter 8 ↗