IMO, it suggests
too much to be a mono 
I would take the concept of a Moondial and make a different card that uses
and 
Such as Sundial does (for the elements not for the effects).
Thought about that. One of my intended plans is to make it delay for 3 turns when played, and give it a

activation cost to deal damage (and sacrifice the card).
But somehow I feel it's a bit cheesy, and doesn't give a sense of countdown.
Besides making it mono-darkness creates more synergy with other elements e.g.

as well as

Anyway, would you have a better idea to bring

together ?