Bone Fortress

You create a tower of bones that is 10' in diameter and 20' tall. The legs of the corpses gather near the bottom and the arms and skulls near the top. You can use a bonus action to have the tower move up to 20' on your turn. Creatures in the path can make a Dexterity saving throw DC 10 to move out of the way or take 7d8 trample damage or 1/2 damage if the creature is large or larger. The top of the tower can hold up to 4 medium creatures which are behind a parapet of limbs and skulls providing half cover from certain angles. Any creature who climbs the tower or is large and adjacent to the tower at the end of their turn are attacked by the limbs and skulls. They creature can make a Dexterity save against the casters spell DC or take 5d8 damage. 

When the tower appears, each creature within its area must choose to be on the tower or pushed adjacent to the tower. 

At Higher Levels. When you cast this spell using a spell slot of 7th level or higher, both types of damage increase by 1d8 for each slot level above 6th.