Multipliers
Difficulty | Dmg Done | Dmg Taken
------------------------------------------
V.easy | x 2 | x 0.5
Easy | x 1.5 | x 0.75
Normal | x 1 | x 1
Hard | x 0.75 | x 1.5
V.Hard | x 0.5 | x 2
Survival | x 0.5 | x 2
Not sure about the # for NPC's HP increase.
It also affect some other things, like the Healing speed/rate, spawn rate of legendary enemies and other similar effects. I don't have hard numbers for those though. There are no new mechanics on higher difficulty.
