Finally getting around to double checking some of my old stuff feels nice. Purging Flame was renamed to "Aura of Shattering" when I added it to the

Quote Originally Posted by JNAProductions View Post
Purging Flame good.

As for the Rogue bit, it's the ol' Haste trick. Use your main action to Ready an attack, use your bonus action to attack normally. When the Readied attack triggers, it's not your turn anymore, so you get Sneak Attack again.
It seems all the notable published powers available to high level rune priests have similar effects. How's the following?

Embers of Wrath
By 17th level, your hunger for ruin lights your weapon and burns your enemies. When you hit a creature with a weapon attack on your turn, you can scar the first creature with celestial flames and immediately make an additional weapon attack against a different creature within reach. On a hit, the second creature is also affected by the flames. For one minute, each creature affected by the flames takes 1d10 fire damage at the start of each of its turns. At the end of each of its turns, an effected creature can make a Wisdom saving throw against your spell save DC, ending the effect on a success.

Also, each creature affected by this feature also takes 1d4 fire damage each time it takes no damage from a spell or weapon attack that dealt damage to a different creature affected by this feature.

Once you use this feature, you can't use it again until you finish a long rest.

Set up to go off on a hit to avoid wasting it, but otherwise similar to the 5th level Rune of the Embers of Wrath runepriest power in 4e. Hoping the result is legible, but I think I need someone else to confirm.