once zed gets assasin dagger his ultimate does 25% missing health dmg

No it doesnt. it does a % of the 25%. So lets say nightfall deals 300 due to plot magic. Zeds ult would deal half of that damage to you if it did say 50% of damage dealt you would take 150 damage not the full nightfall damage again. I dont know how much his ult does atm but its just an example.

its actually not that great on zed. and the item doesn't work like that