level 4 skills

[Mustafa13]

Are they gained randomly from all mobs in any floor of hellmouth cave? or is it only mobs on infernal rift and all subs & bosses?

Any info would be appreciated, ty

 

[UnKnownAngel]

I'm sure I have read somewhere that it is something to do with the infernal rifts, and you have to find the upgrade for each individual skill