OnlyDarknessSurvives (or ODS for short) is a small mod that adds the ability to use magic in (currently) two forms; attack and movement.
There is a new drop from every existing mob called "Shadow Orb" which is used to create the "Table of Shadows" and "Shadow Fuel". Put Both Shadow Orbs and Shadow Fuel inside the Table of Shadows and you could potentially be given "Essence of Darkness".

Essence of Darkness is used to create "Darkness Armor" which will allow you to explore dark places without being inflicted by Darkness, however, Ṣ̶̑t̸̗̊ȧ̵͈y̶̻̌ ̵͍̔a̵̪̿w̴̠͒ȧ̶̹y̷̼̐ ̶̜̚f̵͕̀r̴̮̄ö̵͍m̴͈̌ ̵̥͌t̴͖͠h̶̦́e̶͈̎ ̷̳̏L̷̤̎ȉ̴̞g̶̬̒h̶̙͠ṫ̸ͅ when you have the armor on.

One final vital thing you need to know about the mod. Press G to open the "Magic List".