There are 3 main terrain modifier spells: landbridge, flatten and erode. Other spells can modify the land aswell, such as volcano, earthquake and armageddon.

 

Landbridge is used to create bridges of land (either to expand or to reach/explore unreachable areas. It's the most versatile of them all, as it can create land, hills, lower land, and other.

Flatten can be used to create a squary chunk of land, or, if used on water, erode all the affected land to water (even if the land is very high). If used on hills, it will create that hill plain and squary, expanding that hill in area.

Erode will create water if casted in land close to water, or very thin.. If casted in places not close to water, where the terrain is not that thin, it will almost have no effect. Erode can be devastating when casted in hills, as will massively affect them, and any buildings (and units) on it.

 

Volcano interacts with land as it will create a cone and then will create a small hill on the center. Also, the lava will create land if it touches water.

Earthquake can create land (as the lava touches water) or erode it.

Armageddon will create a land arena, modifying the land in the point where it was cast.