The gladius is a Slayer monster, requiring 81 Slayer to combat. Described as 'highly mobile', it is a member of the Order of Ascension.
During combat, gladii use a special attack in which they charge towards the player and stun them, seemingly at the start of the battle, and then throughout as damage is dealt to them. The movement of the charge is roughly an L-shape, like that of the Knight piece in a game of chess. This charge can be cancelled entirely by doing one of two things, the first being to stay close to the gladius. If you are immediately next to a gladius, it will jump backwards from you in an attempt to do its charge attack. If you do manage to keep up with the gladius as it moves, it will not charge. The other is very simply to safe spot it. This is most reliably done using tables that can be found in rooms throughout the dungeon, however, they can get around them at certain angles due to the charge having a potential L-shape.
They are weak to Ranged but despite being Magic creatures, they have a very high defence against Magic.
Note that combat familiars, including Beast of Burden familiars, tend to take heavy damage from the Ascended. They usually die too quickly to be very useful. Familiars with other uses, like Bunyips and Unicorn Stallions, may be better choices.
When facing gladii, it is recommended to use Anticipation as often as possible, and Freedom when they stun you. This helps because whilst the gladius is trying to stun you, it takes three times the damage.
Due to the fact that they teleport constantly, Fragmentation Shot is extremely useful here as whenever they teleport, they'll suffer the bonus damage. A great opening strategy to fight Gladii is to stand as far as possible away, and use fragmentation shot. The Gladius will almost always rush towards their opponent at this range, which upon a successful fragmentation shot, will result in one hit at normal damage and four hits at 2x damage, due to the fact that the monster has moved. Fragmentation shot should be used at key points detailed in the previous paragraph to continue causing this effect.
When attacking, the gladii tend to jump backwards after being hit for around 1200 damage in a single hit, so if a player wants to follow it, then these hits should be watched out for. For example, hitting very frequently (e.g. using Rapid Fire on them), results in them teleporting away almost instantaneously. Hitting less then 1200 in a single hit results in them moving backwards. They will not stun if they can't reach you or haven't been dealt any damage even if at a fair distance upon the first hit; however, if you move too far from them and they have been dealt damage (even from bleeds), and can stun you, they will stun you instead.
A gladius is a type of Roman sword, which were sometimes wielded by gladiators, which the monster resembles.
Gladius are one of the few monsters capable of stunning their target, the others being Nex during her ice phase, Vyrewatch (until meeting Vanescula in The Branches of Darkmeyer), the Kalphite King, and most ability-using monsters.
When stunned the first time by a Gladius, the player is stunned and bound for 3 seconds. However, upon further stuns as long as the player keeps attacking nearby Gladii, they will only have a 1-2 second stun with a 3 second bind. This is most likely a glitch.