There are many Nightlords to challenge in FromSoftware's newly released co-op roguelike Elden Ring Nightreign, but before you can hunt them, you'll first need to target and defeat the game's first major boss: Gladius, Beast of Night. This three-headed, Cerberus-like wolf may be positioned somewhat as a tutorial boss as you learn how to play the experimental Elden Ring offshoot in the Tricephalos Expedition, but make no mistake: it's not an easy battle, and treating it like one will inevitably lead to your quick demise.
With that said, Gladius is still arguably simpler to take down than the Nightlords it's gatekeeping, and once you and your allies understand each of its moves, how to avoid them, and when it's best to go on the offensive yourself, the fight becomes a lot easier. Taking advantage of the boss' damage type weakness and using Nightfarers optimal for the battle helps, too (though you can be successful with any character).
