Enlighten is a support gem that has no modifiers, and whose only function is having a mana multiplier less than 100%. This is primarily used to decrease the amount of mana reservation of linked skills. This effect is multiplicative with the effects of Reduced Mana Reservation passive skills. Though Enlighten can also be used to decrease the mana cost of linked active skills, Reduced Mana is better for that purpose.
Although the table below shows the effect of the gem up to level 10 Enlighten can only reach level 3 without being corrupted or socketed in an item with the modifier +1 to Level of Gems in this item.