[Daniel V]

Eval scripts are where things of this nature are handled, applying numerical modifications and such. You CAN run things in eval rules, but that's used for checking if the thing has certain parameters fulfilled after you have added it to a hero. Ability Focus is a good example. It applies its effects in an eval rule, where is also assigns a tag to the selected ability. Then it checks how many of that tag is present on the ability and if there are 2+ it returns invalid, and shows the error message of the eval rule. Generally you won't put things in eval rules, but there are cases where it's appropriate.

When you're in the tab of the kind of thing you're trying to create in the editor, hit "New (Copy)" in the bottom left. That will pull up a list of all the things of that type, which you can load. Things in grey are read-only, but you can copy the scripts. Things in black you can edit and save, setting a new id, which is useful for making variations of items, or having a place to start. Keep in mind they'll be the same thing type though, so you can't for instance load a belt of giant strength and change it to armor, you have to make armor and copy the script from the belt. Looking at existing things and a lot of trial and error are how I learned!

Good luck on the energy resist!