You need to be active GM for 2+ years, then you start to cast more and more darkness around you.
GM status is like the magic rings in the LoTR, it brings fancy powers like ability to disappear, but it
also brings doom to the wearer after years of use.
But another way is to make a copy of gamma gulp beer proto and play with Color and Light settings
in Object Editor. I think color has to be something like 0 0 1, because 0 0 0 is default not black.
Then spawn this item and put in item2 or item1 slot. You can also manipulate that through scripts
on any existing item:
class Item
{
(...)
int8 LightIntensity;
uint8 LightDistance;
uint8 LightFlags;
uint LightColor;
(...)
Don't forget to enable ITEM_LIGHT flag on the item.