Here is my homebrew variant of the [*true strike*](https://www.dndbeyond.com/spells/true-strike) cantrip:

> 
> Lethal Strike
> =============
> 
> 
> *Enchantment cantrip*
> 
> 
> **Components:** V, S  
> 
> **Casting Time:** 1 bonus action  
> 
> **Range:** Self  
> 
> **Duration:** Until the end of the current turn
> 
> 
> When you roll a 19 on a weapon attack, that attack is also a critical hit, then this spell ends.
> 
> 
> 

This spell is brewed for my players' Arcane Trickster, who would like a combat cantrip that will enhance his Sneak Attack. He does not like *true strike* because it wastes one full turn before he can attack, so I made this one require a bonus action instead.

I'm worried that making it require a bonus action instead of an action will make this overpowered.

**Is this homebrew spell overpowered as a cantrip? If yes, what is the suggested spell level for this? What can be modified so this can stay as a cantrip?**