Here is my homebrew variant of the true strike cantrip:
Lethal Strike
Enchantment cantrip
Components: V, S
Casting Time: 1 bonus action
Range: Self
Duration: Until the end of the current turnWhen 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?
Best Answer
It's about on par with Green-Flame Blade, a spell published in the Sword Coast Adventurer's Guide (at least for characters with only one attack).
Lethal Strike's expected bonus damage for a sneak attacking level 3 rogue is 0.575 damage (5% * 1d8+2d6 (11.5) = 0.575), considerably less than Green Flame Blade, which deals damage equal to the caster's ability score modifier (we'll say 1-3, since the rogue casting it most likely has a decent Int score), albeit to a second target in melee range.
I'd counter the argument that others have made about it being zero cost to cast this. Rogues have a huge number of uses for a bonus action, and this is competing with all of them. For example, if the character were instead dual wielding short swords, rather than using lethal strike with a rapier, as above, their damage instead increases by 1d6-1 or 2.5 * 1.05 = 2.625 (-1 from going from d8 to d6 on their main hand attack, with a 5% crit chance), assuming they hit with both attacks.
All that aside, your duration doesn't fit with other published spells. Every spell I'm aware of that buffs the next attack is written with the following duration:
As long as the only character to ever get this spell is this rogue, I don't really see a huge problem with it. If a character with a less crowded bonus action economy learned it, however, it could be problematic.