FAQ

Terrain is a subtype of land.
There are two ways to play a terrain:
1. Use a land play.
2. Pay the mana cost.
Either way you can play them only when you can play a land and it never goes on the stack.

Terrains have two values printed on the bottom right. First number is amount of structure counters to put on it when it enters the battlefield. During your untap step, put a structure counter on it. You may not put more than the second number this way.

As a special action, you may move a creature you control onto a terrain you control, or remove it from a terrain. This creature now occupies the terrain. Do this only any time you can cast a sorcery. A creature can occupy no more than one terrain at a time. A terrain may have any number of occupants.

Creatures may attack terrains directly. For each damage dealt to a terrain, remove a structure counter from it. Note: creatures attack and block per usual.

As a state-based effect, a terrain with no structure counters is put into its owner's graveyard. When a terrain leaves the battlefield, remove all creatures from it; they stay on the battlefield.

That's it for now...

Hm. You say you can play Terrains by using a land drop, bypassing the cost? So isn't this way better than Forest? As in, far closer to obsoleting basic lands than would be allowed?

Maybe, theoretically. haven't tested them. For now I'll err on the side of playability.

This feels like it tries to combine too many ideas into one. For most instances "land that also has a mana cost", "card type with structure counters" and "permanent type representing a place on the battlefield individual creatures can maneuver to" are individually enough to create a new card type (and in the second case it did!) - but you haul them all together AND add the rules to the existing card type land. That's a lot.

The problem of combining those mechanics is that it also creates questions about how the individual parts interact and why (not) e. g. I expected there to be a connection between creatures occupying the Terrain and those blocking for it.

Moving creatures around also seems awkward since it is so unimportant if you have only one (compared to enchantments) - except if you e. g. cast an instant that creates tokens.