2014-10-01Skull Modifiers for Heroic missions: these are the modifiers applied to make that mission or strike harder. An example of this is Arc Burn which is currently associated with the Weekly Heroic Strike.

Here is a list of all current modifiers.

Skull	Effect
Juggler	No ammo drops for your equipped weapon.
Anger Issues	Minions of the Darkness display heightened aggression.
Angry	Minions of the Darkness won’t flinch, even after massive damage.
Arc Burn	Arc damage from any source is greatly increased.
Solar Burn	Solar damage from any source is greatly increased.
Void Burn	Void damage from any source is greatly increased.
Lightswitch	Minions of the Darkness deal much more melee damage.
Nightfall	You will face a relentless army that has never known defeat. If all players die, you will be returned to orbit.
Heroic	Enemies appear in greater numbers and are more aggressive.
Epic	Heavily shielded and highly aggressive enemies appear in great numbers.