The Halo franchise has a reputation for providing players with great villains to battle. Creating a good antagonist is one of the harder parts of constructing a story. If they interrupt the story too much, they become less threatening to the protagonist. However, if they never make an appearance, they become little more than a myth. One of the major enemies of the UNSC, Atriox, the leader of the Banished, is close to having the latter problem.
Atriox made his first appearance in Halo Wars 2, where he was introduced and then took a step back from the main story. While he was always a presence, he was rarely involved with the events in the game, although Atriox was available as a character in multiplayer mode. Atriox was present in Halo Infinite, but once again, the player never got to fight him.
Atriox was the main cause of the UNSC's defeat and the destruction of the Infinity. He is first shown in a cutscene defeating the Chief and throwing him into space. Once again, players are only ever shown how powerful he is and never get to truly experience it. This isn't the first time the series has done this with their antagonists. The original trilogy had the Prophet of Truth as a villain, but he was less of a warrior, so his lack of a combat encounter made sense. Atriox is a warrior and a front-line fighter, so not engaging him in the opening makes him feel more like story fodder than an actual threat.
A good example of a constant threat that makes physical appearances in games would be the Reapers from the Mass Effect series. They are consistently present in the games, but their direct involvement varies. Their actual power is showcased in the first game wherein one of them manages to take out several ships with little difficulty. Another example would be the bosses in the Dark Souls series and how they are presented. The series' lore is often hard to find, but the power and strength of the individuals are mentioned often enough for players to pick up on it. The player is constantly told how powerful the heroes of the past are, and then they get to fight them and discover how accurate the statements were.
Atriox suffers as a viable villain because he is a warrior and his interactions with characters have been physical. The player sees Atriox defeating soldiers and are constantly told how strong he is. This sets up Atriox as a future opponent for the player to fight. Ideally, the fight between Master Chief and Atriox would have been playable. If this had been the case, then players would have felt more connected to the threat as they found themselves being beaten down by a powerful opponent. As it stands, it is simply one more story about the character with no involvement for the player.
Antagonists need to be both a threat that the player dreads coming into contact with and an opponent that offers an enjoyable battle in the future. The defeat of a major villain is the end goal and needs to be built up, but Atriox is in danger of being no more than a legend. Hopefully, the next game finally allows players to engage the leader of the Banished and find out just why he is so feared.