Where does Void Coefficient come from in TFT Set 17?

Void Coefficient is a Factory New crate item.

Void Coefficient

Each cast reduces Mana Cost by 15% (to a minimum of 10)

graves

Each cast reduces Mana Cost by 15% (to a minimum of 10)

Mana

The unit-level resource that gates ability casts — champions cast once their mana bar fills, so low-mana units cast more often than high-mana ones. Spear of Shojin (mana per attack) and Adaptive Helm (amplifies Mana from all sources) push casts faster, alongside starting-mana augments. Blue Buff is no longer a mana item in Set 17 — it's now an AD/AP amp.
On Cast

This effect triggers the instant a unit casts its ability. Items like Lich Bane and Innervating Locket reward you for landing as many casts as possible.
Cast Frequency

How often a unit casts its ability. Most damage, healing, shielding, and crowd control on a board comes from abilities, so squeezing extra casts per fight usually decides who wins the round. Two main levers — Mana governs the cost, refunds, and starting pool that gate every cast, while Attack Speed controls how fast the unit auto-attacks (each attack generates Mana).