See Archdruid Gessavine.

Soo the other things to be invoked are clear, but what is a nether card?

Doesn't this go infinite when you get it to go off once? Since it does something to a nether card; re-invoking nether? (Set currently has two other cards that nether; but I agree we've not seen any nether type/subtype/supertype cards yet)

I was planning on making it a supertype/type/subtype (haven't decided, yet)... they don't do that kind of stuff super often, but it seems the simplest.

I don't think this goes infinite. You only invoke nether when an effect says you do (this card doesn't invoke anything, only fetches nether cards) or when you play a nether card (this puts it in your hand, allowing you to spend mana to get value, but unless you've somehow made your cards cost nothing, or you have infinite mana, this can't go 'infinite'. depending on the turn and your deck composition, this could very well cast a lot of cards...

In other words, it's a very efficient engine that your opponent will want to kill)