Upgrading Dark Seal to Mejai's Soulstealer feels pretty awkward. First, you have to invest gold, but then, even if you're already at 10 stacks, you have to get a few more to get the bonus effect. This feels even worse when you would have had more than 10 stacks, but had been capped by Dark Seal.
STACKS FOR MOVEMENT SPEED :  ⇒ 10
While always a big risk, Mejai's cooldown reduction reward rarely benefitted users who often hit the CDR cap far before reaching max stacks. Movement speed fits better, turning the back-and-forth risk vs. reward more tense as you try ascend to glory.
TOTAL cost : 1400 gold (unchanged)
BUILD path : The Dark Seal + 1000 gold
UNIQUE passive Do or Die: Grants 4 Glory per champion kill or 2 Glory per assist, up to 25 Glory total. Upon dying, you lose 10 stacks of Glory.
UNIQUE passive Dread: Grants 5 ability power per Glory. Grants 10% movement speed if you have at least 15 Glory.