This spell can flatten any rugged land or even make more land near a coast (or delete it if you're not careful enough). 

It depends where you cast it:

Casting it on water will drown nearby land.

 

Casting it on land will flatten nearby land to the height of the land where you clicked with your mouse. 

This means you can either create water, create hills or erode hills with this spell - or anything inbetween that.

This spell can be annoying to the caster specially when you are trying to build land from it and you want to use it near the shore (close to the water). Some terrain types (landscape styles) can be tricky, and what you think is land is actually water, so you end up eroding land instead of creating land. This can be avoided by having knowledge or just not casting too close to the water when trying to create land.

 

Flatten is also really good to drown shore buildings (like boat hut) or to destroy a row of buildings (or important buildings such as training huts) by casting in a hill close to them.

 

You will obtain this spell in singleplayer's level 14, and you can store a maximum ammount of 3 shots at once.

 

This spell is probably used in half of online games, but when it's on it's rare to be a priority charge spell.

 

Some nice tricks or combos can be done with flatten, so check the tricks section!