This DLC includes the following items:

①Idea--A legendary weapon exclusive to the Caster.
Equip Idea to obtain the following effects: "Increased Runes Gained", "Increased Crit", and "Increased Max HP".

②Voidstone--A rare Magistone.
Set it in a weapon slot to obtain the following effect: "Increased Stun Resistance".

More details on the official game page of Caster's Special Weapon & Voidstone Set ...