Knights-Arcanum are mages of the Valedictor Temple of Stormcast Eternals charged with the recovery and protection of lost knowledge but they are also powerful wizards that can command the storm in battle. 
Knights-Arcanum are sages and loreseekers of the Valedictor Temple to which when promoted is a permanent sentence and a Stormcast of the temple can never join or even form a close bond outside their ranks. They bear knowledge that is both vital and terrible which they are forbidden from forgetting. To aid in this they use mnemonic gemstones linked to Hysh which aid in storing large amounts of knowledge but at the cost of dissociating from themselves. Thus they discard their previous identities and replace their names with various titles. They also tend to be quite eccentric or in some cases outright mad with weird quirks such as a tendency to speak in riddles or attempting to follow strange hunches far from their command which tend to irritate more hardline commanders. 
In battle they wield meteorological sorcery empowered by their forbidden knowledge and the magic of Realmspheres. Their ability to unleash fist-sized hailstones and gales of storm magic upon enemies mean their fellow Stormcast Eternals tend to be forgiving towards their odd behaviour. [2a]
The magic of most Knights-Arcanum are based on the study of arcane meteorological phenomena of the realms however many different sub-schools of magic exist in the Valedictor temple. This is in contrast to the soul-magic of the Sacrosanct Chambers. These wizards use their powers to summon sleeting hails and rains of blessed starwater that assault the corrupt while purifying their brethren. These spells are also affected by the realm they are in for example rains of Chamon will consist of mercury droplets while gales in Ulgu are followed by banks of fog.
Knights-Arcanum are armed with a Valedictor's Stave and clad in Thunderstrike Armour. Their armour is covered in scrolls and tomes collected from all across the realms, the arcane secrets within protecting them from raging Endless Spells.[2b] Hyshian Gemstones are embedded into the artifacts they carry which allows them to expand their mind to allow perfect recall at the cost causing a level of madness.[2a]
- 1: War at Amberstone Watch (booklet) - Knights-Arcanum, pg. 13
- 2: Battletome: Stormcast Eternals (2021)