From The Heroes of Might and Magic III wiki

Class	Earth Magic
Basic cost	8
Duration	Unlimited
Level	2nd

Places spots of quicksand on the battle map. These are invisible to the enemy unless he has a unit that's fighting on its Native Terrain

Level	Effect
None	4 patches
Basic	Cost reduced by 2
Advanced	6 patches
Expert	8 Patches

[edit] Comments

Even though many see Quicksand as a useless spell it does have some charm, especially during a siege. With Expert Earth Magic it can cause a walking unit to become stranded and say, not be able to advance a broken drawbridge. It can also be entertaining to play Quicksand and Land Mine with your enemy when you have the Shackles of War equipped for a final battle.