I think the big question is if the secondary attack that you get from the "Hit:" string is per target hit, or not.
I.e. Cast this spell, you hit 5 enemies, they take 2d8+8, say, and get the ongoing damage. Do you then make 1 secondary attack, or 5 secondary attacks? You did the rest of the text under 'Hit:' 5 times, once to each hit target... why not the 'Make a secondary attack' too?
The sustained effect is pretty nice, but probably only a single use of the secondary power per round; but if you really get to make X secondary attacks int the initial round when you hit X enemies with the primary attack... That's a freaking Doomsday weapon. It makes for a nice visual, too...
The party is surround by many powerful undead; vampires, wights, even liches! The Warpriest holds his holy symbol high and screams "You want a fight? By Pelor, it'll be a Battle of Pyres!" White energy explodes from his symbol to the surrounding enemies, and many of them flinch in pain, and begin to glow white themselves. The leader chuckles, "You'll have to try harder than that, these are not mere minions..." The cleric replies, "I'm not done yet." The creatures who are glowing suddenly flare up, massive bolts of light leaving each of them, to impact another glowing corpse with holy light, far worse than the initial wave. The rest of the party lays into the screaming crowd. Even after that second wave, the cleric continues to concentrate heavily, and white bolts continue to fly between the glowing creatures, with a flash of light and a wail of pain.