So the idea is to change nightfall/eclipse to cost darkness quantum per turn (similar to flooding) but give greater benefits of maybe +1 more damage and hp or just make it able to stack.
For example, if one darkness card cost 2 darkness at the end of each turn for a bonus of +1/1 you could have 3 down for the cost of 6 darkness each turn for +3/3.
Agh i hope i explained this right. These numbers aren't what im suggesting but the concept.!