An Idea I had for transforming lands, may need balancing

Hmm. Somewhat similar to Earthform / Lurk. Which is a good thing. I probably wouldn't make this an actual Forest, though; being searchable by Wood Elves and Misty Rainforest and so on may be a bit too good.

Oh, good idea. Compared to Gargoyle Castle it seems about right. Double faced cards are an excellent way to do these one-off animations. In fact, they could do temporary animations like treetop village, but there's less reason there.

Edit: The size may be slightly too good, I'm not sure. I agree being a forest may be too good.

Hmm... is the lack of a in the second ability intentional. It's a bit odd, but I get it if the vigilance connection is the point. Still, this thing is going to tranform as soon as you have land number 5 on the table and your opponent's turn is ending.

I agree making it a forest was too strong.
At it is slightly harder to cast than Ironroot Treefolk which was the card I was thinking of when I had this idea.
As to the lack of a in the transform ability, both that and vigilance were meant to convey the favour of a long dormant protector being awoken by a threat to it's domain and surging into action (ie: you can activate it and block with it that turn rather than having to wait a turn for it to untap)
Having just written the above sentence though, I can see having tap in the ability cost as the effort to rouse itself from its long dormancy....hmmm
What do you guys think? tap or not tap?

I think it needs to be significantly harder to cast than Ironroot Treefolk. Not taking up a card slot in your deck is huge.

I think the design could work either way with : see Stalking Stones for perhaps the most similar card to this printed so far. That doesn't need to tap.