Curse of Silence

A target you can see must make a Wisdom saving throw or they are Cursed. 

While Cursed they are Silenced.  They cannot speak or cast any spells requiring a verbal component.   

They can end the Curse with a successful saving throw, or with a Remove Curse spell.  

In D&D 5th Edition, the majority of spells, particularly those not in the Cantrip category, require a verbal component. Specifically, only about 24 out of the 531 spells do not have a verbal component, and most of these are low-level spells. This means that approximately 507 spells have a verbal component. 

Ironically, the target cannot cast Remove Curse on themselves because that spell has a verbal component.