Each player character has a number of dice called Endurance Dice (or often just Endurance). An Endurance Die is a die that you roll as part of using various Power Set features. You have a maximum number of dice equal to twice your Proficiency Bonus. When you roll an Endurance Die, it is spent and is no longer available. You regain all spent Endurance Dice after completing a short rest.
At 1st level, your Endurance Dice are six-sided (d6). At 4th level, it becomes a d8.
Many Power Set features benefit from adding one or more Endurance Dice to them, usually for damage. The general rules, which may be overridden by specific features, are:
Unless otherwise noted in a feature, you cannot add Endurance Dice to an attack roll or to the damage applied from a feature that prompts a Saving Throw. Some features require that you roll at least one Endurance Die in order to use them. If an attack uses neither an attack roll nor prompts a Saving Throw, you cannot apply Endurance Dice to it (for example, Flares).
You can apply Endurance Dice in this way only to features provided by your Power Sets or Feats, and for any attack with which you have proficiency. There is no limit to the number of Endurance Dice that you can roll at any one time, unless a specific feature indicates otherwise.
You usually do not need to spend Endurance Dice simply to use any weapons or equipment, even those associated with one of your Power Sets. In the case of equipment, you may need to roll one or more Endurance Dice to invoke special maneuvers or shots. This represents any mental or physical effort involved in pulling off the stunt.
In City of Heroes, some abilities are Toggle Powers. These Powers are switched on or off, and once activated will periodically drain an amount of Endurance. The CoHd20 equivalent of a Toggle Power will require you "set aside" one or more Endurance Dice. When you set aside an Endurance Die, you must take one die that is otherwise available to you and remove it from your pool of current Endurance Dice. That die is not spent, but you are unable to roll it as long as you have the "Toggle Power" feature active. If you deactivate that feature, the Endurance Die you had set aside is returned to your pool and can be used normally.
You can normally deactivate a "Toggle Power" feature on your turn without costing an Action or Bonus Action. When you do so, any Endurance Dice set aside for that feature immediately become available for use that same turn.
If you are subject to a condition that causes the deactivation of any "Toggle Power" feature, you are recouped any Endurance Dice that were set aside for it immediately, even if this does not happen on your turn. Conditions that will force the deactivation of all such features are incapacitated, paralyzed, petrified, and unconscious. Being stunned (or any other status) does not automatically force the deactivation of a "Toggle Power" feature, unless the feature that prompted the condition indicates otherwise.
Some Power Set features target multiple creatures in an area-of-effect region, or as part of a cone attack, or in some other location-based targeting rather than requiring an attack roll that is countered by a creature's Armor Class. Often when you use one of these features, you roll one or more Endurance Dice to boost the damage, increase the DC for any Saving Throws associated with the feature, or for some other reason. Whenever you do this, for each Endurance Die you roll, you may select one creature that would otherwise be subjected to the effects of that feature, and that creature will be considered unaffected as if it wasn't present in the target area. This will allow you to moderate the amount of "friendly fire" your area-based features inflict. You are not required to do this but your teammates will appreciate it.
Whenever you select at least one other creature to be considered unaffected by one of your features in this way, you automatically become unaffected if you so choose. Exempting yourself does not cost a specific Endurance Die on top of what you apply toward that other creature.