If you're looking for something that is purchaseable with DoN crystals that extends detrimental spells, like your current cloak, there isn't one. If you just want one that increases the damage from DoTs, they do sell Burning Affliction ones.
I believe all the decent (works in full effect up to level 70) detrimental extension focus effects end with the words 'of the Dragorn.' There may be some out there that I am not aware of, though. I was trying to get enough LDoN pts to get
Jagged Gravestone of Affliction until Elorian pointed out that it would probably be easier to get
Facade of Fright from MPG Fear trials.
Again, there might be some droppable items from DoN that have similar effects, but I do know that the DoN merchants do not sell anything comparable for crystals.