
Xah'Rith attacks by making frozen corpses rain from the sky and if you get too close to him, he will push you away with a frozen breath attack. He always appears in the Raise the Catapults section of Stonefort which means you can use the Stonefort waypoint and then head to the right and up the stairs.
The Key of Terror can be dropped by Xah'Rith the Keywarden in Stonefort in Act III. He also will use his whirlwind blade attack on you.
Sokahr has the missle dampening, vortex and mortar affixes. Any level of character or Follower can wear one.The Key of Hate can be dropped by Sokahr the Keywarden in the Dahlgur Oasis in Act II.
There is no level requirement on the ring making it an ideal twink. Your Followers can wear a HR, but not two at a time. Initially during the RoS beta it was possible to wear an imported D3 version along with a new RoS version, but this was a bug. The HR is unique equipped so that each character can only wear one HR at a time, whether the D3 or the RoS type. It can be Salvaged to destroy it and retrieve a Fiery Brimstone (D3 ring) or Forgotten Soul (RoS version). The HR is account bound and can not be sold or traded. Thus a Diablo 3 Hellfire Ring on a Follower is worth +7% Experience to the player, and a RoS HR is worth +9% Experience. Followers can equip the Hellfire Ring as well, but just as with Magic Find and Gold Find on followers, they only share 20% of the bonus experience to the player. The main perk of the ring is the substantial increase to Experience gain, and the goal for most players is to create a ring that’s got decent stats as well, so they’re not giving up too much DPS by using the Hellfire Ring instead of a Legendary or Rare ring.