Pretty cool change but if you dont want to have NF again useless then you need to make its duration at least 20 seconds.
To make it worth EVEN with the 20 seconds you will need to rework also sepsis which is not something great.
What i am saying is that every covenant comes with a packet of things and the total is what counts. If necro and venth offers so much then the other 2 should offer equivalent. With 10 secs duration and sepsis as it is NF is a no-go. With 20secs duration and sepsis as it is NF is a go if you want cool shapeshift and other than that i dont see a reason even bothering with it if you are necro or venthyr