This Ichor was crafted by an old colleague of mine: Thule Ravenclaw. He was a young, promising mage before the great war, but when the Lich King rose to power, he betrayed his brethren and allied with the Scourge.
I did not realize he still lives, nor that he leads the Rot Hides in their forays against us!
I know many of his spells, and can help concoct a potion to counter them. Take this scroll back to Apothecary Renferrel. It has the incantations needed for the potion.
This scroll is laced with powerful magic. I've never used such forces in a potion ... it will be a powerful mixture, and should serve you well should you attack Thule in his stronghold on Fenris Isle.