Being one of the most beloved Sci-Fi gaming franchises of all time, it's only natural that Halo has some of the most iconic aliens in all of gaming. Halo's primary antagonistic force, The Covenant, is made up of multiple different alien species, each with a distinct and bold design that's now immediately recognizable from the Grunt's triangular methane-filled backpack to the Jackal's oval arm shield. But even The Covenant might not be considered Halo's greatest threat.
Over the last two decades, there have been several different enemy factions in Halo. While The Covenant is probably the most iconic enemy force in Halo, appearing across the entire franchise, they're not the only enemies fans have faced off against, and they might not even be the most lethal. For many fans, there's one clear winner when it comes to the most dangerous Halo enemy, and that's The Flood.
It's Hard for Halo to Top the Flood
Simply put, The Flood is one of the most iconic enemies in all of gaming, and a lot of that mainly comes down to its first reveal all the way back in 2001's Halo: Combat Evolved. About halfway through the first Halo, the player stumbles upon an underground Forerunner structure. Upon entering the facility, the Master Chief comes into contact with an incredibly paranoid UNSC marine who even begins to open fire on the Chief if he approaches. With the marines being close allies throughout the game up until this point, the player is likely to know that something is wrong here.
A brief cutscene then begins to play as the Chief approaches a marine's body. This cutscene shows first-person helmet-cam footage from one of the marines. Though the player can't really see what's going on, they can immediately sense how chaotic and intense the atmosphere is, with marines screaming out in pain and frantic cries to retreat being bellowed. The player doesn't yet know what's waiting for them below, but they know it isn't friendly. What proceeds is one of the most intense missions in the original Halo, with the player desperately trying to leave the facility while they're being hunted by endless hordes of parasitic alien lifeforms known as The Flood, a being which is capable of reanimating the dead corpses the player finds along their way.
The initial Flood reveal in Halo: Combat Evolved is still widely considered to be one of the most memorable moments in the entire franchise, and it's a moment that the rest of the series has been compared to. The Flood then continued to appear across the original Halo trilogy, and for many fans, its appearance is usually a highlight of the game, adding a compelling sense of horror to an otherwise empowering, action-packed space-faring adventure. Even though some missions revolving around The Flood aren't the best, with Halo 3's "Cortana" being the obvious example, their presence in the original Halo trilogy is a core part of its identity, and for many fans, their lack of appearance in 343 Industries' Halo entries has been a big reason why they often haven't felt like Halo.
But that isn't for a lack of trying on 343 Industries' part. 343's first outing, Halo 4, brought back The Covenant and introduced a new threat known as the Prometheans, ancient Forerunner warriors that were digitally preserved to survive the firing of the Halo arrays. Though the story behind them was neat, Halo 4's Prometheans felt a little dull and imbalanced to fight against, leading to frustration more than satisfaction. Halo Infinite brought back the Banished from Halo Wars 2, and while they're essentially just The Covenant but painted red, they did offer some variation from the rest of the franchise. But despite its best efforts, 343 Industries hasn't managed to top the original Halo trilogy's Flood, and unless it can somehow produce a unique, surprising new threat, it likely never will.