Life represents your physical well-being in game, and you will die if it drops below 0 unless you are under the influence of effects such as a Heroism Infusion, which allows you to temporarily persist with negative life.
Most characters will naturally regenerate life each turn. The amount of life regenerated can be modified by equipment, as well as talents such as Regeneration Infusions can greatly boost life regeneration for a short time as well. The rate of life regeneration accelerates each turn while resting. Under the default keybinding, you can begin resting by pressing the r key. Larger amounts of life can be restored with various healing talents such as and . Many of these do not function on undead characters however..
Your maximum life will increase by Life Rating * (1.1+(current_level/40)) each time you gain a level.From Actor.lua You will gain 4 life for each point you have in Constitution, including temporary points from gear or talents.
Your Life Rating is determined by your race and class. For example, a Dwarf Berserker has a Life Rating of 12 (Dwarf) + Berserker (3) = 15. When going up to level 5, your life will increase by: 15 * (1.1+(5/40)) = 18.375.
The expected maximum life of a level 50 character will be:
Expected Max Life = Base Life from Class + (85.75 * Life Rating) ... plus your points from Constitution
Note: 85.75 comes from the sum of the series (1.1 + n/40) for n=2->50
NPCs have life in the same way, by using the Life Rating function from above. Additionally, NPCs have a 'rank' value which determines how much their life scales. This is reflected by in-game difficulty of NPCs. Regular NPCs are rank 1. Bosses are rank 4.