The Darkness Within
Magic in this world is a new, elusive art that appeals to many but is mastered by few. As with all magics, it can be broken down into two parts: Arcane and Divine.
Arcane magic is the more confusing of the two. The energies that make this magic work seem to flow through certain people, while avoiding others. Many would-be wizards have spent their entire lives attempting to perform a cantrip, while there have been reports of upset farmers causing huge explosions in their fields. No one is certain how the magic chooses its harbingers, but it seems to be drawn to the more attractive.
Divine magics are supposedly gifts granted by the gods themselves. Unlike arcane magics, the secrets of the divine arts are more about fervent prayer and ritual as opposed to bending the universe to your will, so it is more easily accessible to the masses to learn. That being said, few have the capability to become truly powerful.
Iron and steel weapons drawn in anger for some reason seem to interfere with the ebb and flow of magics, making it rather difficult to cast spells in the thick of mundane combat. Only a priest or sorcerer of great power can truly wade into the thick of battle and successfully cast mighty spells.
In addition to the regular concentration checks, please make note of these changes when blades are drawn:
If trying to cast a spell in melee range of an enemy combatant, a caster must make a DC 25+spell level concentration check. Melee range means if you can hit it, or it can hit you, you are within melee range.
If trying to cast a spell in “Close” range (Up to 25’ away), the DC is 20+ spell level.
If trying to cast a spell in “Medium” range (25’-100’ away), the DC is 15+spell level.
If trying to cast a spell in “Long” range (100’- 400’ away), the DC is 10+spell level.
Over 400’ away does not require any special concentration checks.
Failed concentration checks result in spell failure.
So, to be clear: If metal weapons are drawn and being used or readied for combat near a caster, he/she must concentrate especially hard in order to cast spells.
Spells described in the PHB will automatically be allowed, all other spells subject to DM approval.