First get 2 or more epic *purple lettering* equipment. Weapon or armor it does not matter.
Then after that Compound or combine them together.
You have a slight chance at getting Raging Storm.
You also have a chance to get other legendary equips also.
To get the epic equips it should be by boss battles or the auction shop which will be pricey or you will have to make an epic weapon by combining the blue letter weapons or armor to get an epic one.
Remember to make sure that the equipment you are compounding is the same lvl in other words both lvl 99 in order to get the sword or the other legendary equips.