Arcsteel

Steel alloy containing Ignium . The exact process creates a bluish, very slightly luminescent tint that amplifies when magic is run through the metal. Finished items are generally blued similarly to ordinary steel after production, resulting in the material being commonly associated with a deep, vibrant shade of blue.
In its simplest form, strength (tensile, sheer, compression) goes up proportionally to the amount of magic being conducted, leading to some ridiculously tough ignium-powered armor, among other innovations. Theorized to occur because the molecular structure of the metal naturally focuses magic into hard light, similarly to Noble Metal Infusions . Various alloys were developed, the primary defining characteristics being responsiveness to magic and steel & ignium being used in construction. In the later days of Epacs, arcsteel development was a field of research in its own.
The flexibility of arcsteel alloys despite the seemingly limited ingredients comes from the fact that they are fundamentally created through infusing a durable material with blue quanta: one of the fundamental building blocks of magic based around predictability and set, programmed functionality. The simplest result is to build on what already exists: the strength of the steel. By messing with composition and production methods, other properties (magnetism, weight, luminescence, only tensile strength etc.) can be adjusted through magical input.

On varying compositions & internal structure

Arcsteel gains strength both from hard-light microstructures and molecular bond strength inherently increasing with increased imbuement state. Note that the Magical Coherency Limit being 10µm does not mean that it's the minimum edge size- the actual effective minimum thickness for light-based reinforcement is far higher (~4x as much) than that due to the projections being formed by magical structures.
Various compositions are based around differing micro-structures and their effects. Increased luminescence is essentially making the material less efficient at turning energy into strength- magical energy 'waste' takes the form of light. Tensile/compressive-focused strength adjusts the way that hard light is internally projected in an irregular way- as a result, they tend to be overall weaker than standard uniform arcsteel, but more efficient in their given stress/strain. Due to the Magical Coherency Limit , micro-scale hardness cannot be conventionally focused on. Certain high-end alloys are set to exhude a visible 'haze' of flickering light, which immerses the surface in order to harden it through the charged edge effect. However, this is ludicrously energy expensive. As the net gain in thin edges only comes from the imbuement state increase without hard light structural reinforcement, it's also dramatically less efficient. In fact, the real use of these alloys is the haze itself- the constantly extruded sparks of hard light create an abrasive effect similar to a minature belt grinder. These alloys are generally used in cutting/shaping tools, generally being bolted down and requiring a very large source of nearby energy to power them. Certain weapons also utilize them- the famous Epacsi greatsword reactively pulses its haze alloy as a response to force, minimizing uptime (and therefore energy usage).

On Dilith

Dilith actually disrupts the effects of standard arcsteel. Self-repair enchantments can be etched to replicate its effects, but the mineral itself destroys efficiency due to soaking up magical energy to 'set' its state instead of allowing it to flow through the ignium. This has a knock-on effect of also removing any durability granted by the dilith, as it'd constantly be in a setting state.
As the Epacsi mountains bordered the heavily fire-aligned Molten Desert, the regional ore had (relatively) large amounts of dilith- oftentimes almost 5% by volume (with the ore typically being ~60-70% iron). Arcsteel production was consequently also the primary source of dilith.