Is Dark Seal OP?
With Koreas 5 Dark Seal singed build that has been up and coming ive been trying Dark Seal on Kassadin as well, and it has been working incredibly well. I main Kass and have 500~ games on him and normally do pretty well on him anyway, but building 3/4 dark seals + corrupting potion has made my laning phase feel really easy recently, and makes it so I get to my mid game much sooner. Im not sure if I would call Dark Seal OP, but maybe the increased healing from potions should become a unique passive?
https://s30.postimg.org/8zho5hdzl/kass_games.png
All of the Kass games except the very bottom one I got at least 3 dark seals