This is my take on the possible design for the ultimate Night spell. Just to explain my picture a little more it's simply a dragon. I was thinkin' it be like an opposite to the phoenix like yin and yang, fire and ice, dragon and phoenix. I also made its effects different to that of the Phoenix Fire so they're not exactly the say as the ultimate Day spell. Just to add a but more variety/diversity to the game.
[EDIT] Also, forgot an important part but before the dragon explodes out of the ground and insta-casts a Frost Nova, there's a few second delay where the ground freezes over and slows moving targets.. and then BOOM! Dragon comes outta the ground and frost nova dishes some ice damage.