Weapons are your main mode of attack. Initially, you only have the Stygian Blade available, but additional weapons can be unlocked by spending Chthonic Keys in the Arsenal Room. Unlocking all the weapons requires a total of 24 Chthonic Keys. They can be upgraded during a run with a Daedalus Hammer.
Each weapon has a number of "Aspects" that can be unlocked using Titan Blood. Aspects become available one run after the Rail has been unlocked and the player has collected at least one Titan Blood.
The infernal arms are the weapons the Olympians used to defeat the Titans. The Olympians bound them afterwards to prevent their use, but the arms are supposedly the strongest or some of the strongest weapons that exist in the universe which is why they are supposed to be locked away. Why exactly they appear in the courtyard, is unknown.
Because of their history with titan blood they both react to it when it is collected and seem to have developed some form of consciousness.
"...Stygius, the Blade of the Underworld, must have been among the finest weapons ever wielded, back when it was whole..."
The blade's default attack pattern is a three-swing long combo consisting of a mixture of wide and directional swings. The special creates a small burst around you after a short jump, and leaves you stationary for a short time.
"...It must have been a sight when Lord Hades wielded Varatha the Eternal Spear versus the Titans, driving back those fiends into the depths, together with the help of his Olympian brothers and sisters..."
Repeated long-range stab attacks that can be charged to unleash a spin attack dealing high damage in a wide radius. The special throws the spear, which will damage enemies along its path until it stops. Activating special again recalls the spear, which will deal damage on the way back.
"...Aegis, the Shield of Chaos, predecessor to the very Aegis wielded by Lord Zeus and by Athena, his most favored daughter... the Lord of Thunder defended his brothers and sisters using that very shield then, together, they conspired to drive the Titans back into the lowest reaches of the Underworld..."
The main attack is a single swing that hits in an arc and knocks enemies back. Holding the attack button will block damage from the front, while charging the "Bull Rush". Releasing this will perform a shield bash forwards, dealing damage to enemies hit. The special throws the shield, which bounces between enemies and objects before returning.
"...Coronacht, the so-called Heart-Seeker, is certainly the finest bow ever conceived, and wielded once by none other than Mistress Hera, who stood by side with Zeus, on better terms back then, as they drove back the Titans under a storm of arrows and thunder..."
Ranged attacks that hit enemies at a distance. The main attack can be charged to increase distance and damage, releasing at the right moment for additional damage. The special sprays arrows in a cone in front of you, dealing 10 damage each.
"...What is a weapon if not the extension of one's will to survive, to destroy? The ancient cyclopean forge-masters who created the Infernal Arms in accordance with the Fates' design must have understood this when they delivered the singular Twin Fists of Malphon in secret to the gods..."
Fast repetitive short-range combo attacks using fists in close-quarters combat. The special is an uppercut that hits twice. This weapon is unique due to it being the only Infernal Arm with a dash-special; if the special is used while dashing it will uppercut faster but only hit once.
Unlocked with 8 , once all other weapons are unlocked
"...Least known among the gods who stood together to depose the Titans is the Lady Hestia, reclusive goddess of the hearth, and one-time wielder of Exagryph, the Rail of Adamant; an artifact of metal and of flame so dreadful that the gods themselves abandoned it once their fell work was done..."
Automatic or manual fire (depending on whether Attack is pressed or held) that must be reloaded once all ammo is used. The special launches a grenade to bombard the target area, which takes a short time to arrive but deals damage in an area once it lands.
Each weapon has a hidden aspect based on myths from a culture other than Greece, which requires a secret phrase to 'wake' it and reveal the option. The spear's aspect must be unlocked first; once it is revealed, revealing the others becomes possible. (Paying to unlock the spear's hidden aspect is not necessary.) Each phrase comes from a different character, and does not require that the correct weapon be wielded while interacting with that character. However, several weapons require that you first have a conversation with the same character while wielding the correct weapon, and only after that conversation occurs is the later conversation with the secret phrase accessible. In most cases the character who conveys the phrase is thematically related to the weapon and therefore can be guessed. (It is not possible for the player to enter the phrases manually, Zagreus must learn them diegetically.)
For the spear, Zagreus must have reached the surface and must unlock several aspects of his various weapons. For the others, substantial upgrades of the weapon in question are required. Details appear below, collapsed for those who prefer to discover it themselves, organized in approximate ascending order of difficulty.