After having a few in-game conversations with some GAFers, it's clear some people aren't aware of the effects of Nightmare missions. So here they are!
Only one of the following challenges may occur per mission (each has the same chance of occurring):
No Shields: Every Tenno's shields will be reduced to 0.
Timer: The mission will now be timed, starting at 5 minutes. The death of an enemy will add 5 seconds to the timer. Death does not need to be caused by a Tenno and can be caused by death by another faction.
(If the mission type is Defense, Mobile Defense or Survival, this challenge will be ignored. A new challenge will not be chosen in its place.)
Health Vampire: Every Tenno's health will be reduced by 1.5% of maximum per second rounded down. This wont reduce your health lower than 1% of maximum. The death of enemies, including Infested Runners exploding, will recover 12.5% health for the entire Tenno squad.
Energy Vampire: Every Tenno in the cell will lose energy constantly (approx. 1 per second?). Energy Siphon slows down the drain rate.
In addition to the 3 challenges above, there is also a 50% chance of low gravity (reduced by half, this affects everything including player maneuvers in the air, enemies and loot)
Challenges below do not incur the low-gravity addition:
Ammo Storage Deficit: Lockers do not drop ammo, but enemies still do.
Enemy Damage Buff: Enemies will have innate triple damage buff.
High Level Enemies: Enemies will have a higher level than normal.
Lethal Explosive Barrels: Explosive barrels deal a lethally high damage. (around 1000)
Longer Reload: Reload Times are increased. (possibly +50%)