Runestone and Fiery essence from Hellforge
Aside from Hellforge, there's only 2 areas in game that can drop Runestones and Essences.
There are 6 different types of essences. Ice, Earth, Fire, Water, etc. (cant remember)
x1 Runestone + 1 Fire Essence = x1 Fi Rune
Once you repeat this step with 6 different essences, and 6 runestones you get, 6 of the outcoming runes are cubed into 1 rune, XIS RUNE.
XIS rune has no effect on it's own, but XIS rune cubed with any other rune gives new runeword options.
For example, XIS + EL turns EL rune into ELXis rune and it's a completely new runeword using this rune.
Quote
10% Chance to cast level 32 Celerity on Kill
45% Hit Recovery
+(172 to 200)% Enhanced Damage
(301 to 500)% Bonus to Attack Rating
75% to All Attributes
60% Gold Find
+(3 to 5) to Light Radius
Cannot be Unsocketed
This is ELXis in a weapon. Mostly used as a runeword in a Balanced Knife on a throw assassin as al the stats are insane.