You draw the bones from all corpses within range, forming a carapace of ribcages and femurs. You gain temporary hit points for each Small or larger corpse wholly or partly within range, according to its size, that last for the duration:

As an action on a subsequent turn, you can cause your armour of bones to erupt, expending all remaining temporary hit points. Each other creature within range must make a Dexterity saving throw, taking piercing damage equal to your expended temporary hit points on a failure, or half as much damage on a success. Whatever the manner in which the temporary hit points are lost, the bones shatter and can’t be used in this way again.