No Death is a mod that as the name implies means that no entities can die! excpet for you the Ender Dragon (So you can beat the game) and the Wither
Of course you may be thinking, how do you even get mob loot, Well if you do enough damage in one hit it drops the loot but only once per mob.
Also yes mobs can despawn as normal but damage that's not enough to one shot will do nothing
Good luck and have fun!
The No Death Team
