One of the Dungeons & Dragons sourcebooks needed to have all of its spells updated, as the cantrips in the Sword Coast Adventurer's Guide had confusing descriptions, which allowed players to tailor their use outside of what the creators of the game intended. It's common for Wizards of the Coast to clarify confusing rules in errata that is released online, as this recently happened with the overpowered silvery barbs spell, which some though could bypass the Legendary Resistances of monsters.
In the current edition of D&D, cantrips are weaker spells that can be cast without expending slots. This means there has to be a balancing act regarding their combat strength, due to their unlimited usage. The addition of cantrips is a way of making up for a problem that plagued the early editions of D&D, where low-level spellcasters would quickly run out of spell slots, and were forced to use crossbows and slings to deal damage. The cantrips in the current edition of D&D give spellcasters a more impressive action to use during their turn and can keep them useful, even when they aren't bringing out the big guns, by using magic items or spending spell slots.
The Sword Coast Adventurer's Guide was one of the earliest sourcebooks for the current edition of D&D. The book featured a ton of lore regarding the Forgotten Realms campaign setting, including the different factions that are vying for power. The Sword Coast Adventurer's Guide also included new subclasses and spells, one of which was the Bladesinging Arcane Tradition for wizards, which would later appear in Tasha's Cauldron of Everything.
The bladesingers were infamous in Advanced Dungeons & Dragons for being one of the most overpowered classes in the game, yet the new bladesinger in Tasha's Cauldron of Everything is a balanced subclass. The Sword Coast Adventurer's Guide added four cantrips for the sorcerer, warlock, and wizard classes, which seem as if they were meant for the bladesinger. These spells enhanced or emulated the power of weapons and armor, yet the wording in their descriptions was sloppy, forcing Wizards of the Coast to clarify their capabilities.
D&D Spells That Were Changed (& How)
The four cantrips from Sword Coast Adventurer's Guide have received errata, which can be found in a PDF on the Sage Advice page on the official D&D website. The updated versions of these spells were printed in Tasha's Cauldron of Everything, so it's easier to refer to that book for the spells. The cantrips that were updated are booming blade, green-flame blade, lightning lure, and sword burst.
The booming blade spell enchants a weapon for one round. If the target is hit with the spell/weapon, then they will take thunder damage if they move before the caster's next turn. The green-flame blade spell enchants a weapon with fire magic, allowing it to damage another enemy within five feet of the target upon a successful hit. The lightning lure spell conjures a magical whip that drags enemies towards the caster, dealing damage to them if they end up within melee range. The sword burst cantrip creates a magical shield of swords that damages all enemies around the caster.
The changes to the booming blade and green-flame blade spells are the same. They no longer have a verbal component, the caster must be using a D&D melee weapon worth at least one silver piece, and range now specifies Self as the target. The description for booming blade also clarifies that the target has to move five feet or more for its effect to activate, as the original description suggested that any sort of movement would trigger its effect. The lightning lure spell had its range changed to Self, but this one specifies that it has a fifteen-foot radius. This change to Dungeons & Dragons cantrip rules is meant to clarify how the spell works. The sword burst cantrip also had its range changed to Self, and the description of the spell's damage has been clarified, so that it only affects creatures within five feet of the caster.
Why D&D Cantrip Rules Were Changed
The biggest change to all of the spells altered in this Dungeons & Dragons update was to their range. They all initially had a range of five feet, except for lightning lure, which had a range of fifteen feet. The fact that these spells had a range bigger than Self meant that they could be altered by the Metamagic feature of the sorcerer class. A sorcerer could use the Distant Spell Metamagic ability to extend the reach of all of these cantrips, beyond what they were planned to have. It was also possible to use Twinned Spell to allow the cantrip to affect another target.
Distant Spell and Twinned Spell don't work on D&D spells that have Self as their range. Once the spells were updated, they were no longer affected by Metamagic. The reason why booming blade had the five feet tacked onto its effect is that the initial description could have been interpreted as any kind of movement activating the spell's effect, such as attacking or drinking a potion, when the intention was for it to activate if the target tried moving away from the caster.
The cantrips were also affected by Feats. The War Caster feat lets the caster use a spell as part of an attack of opportunity, so long as the spell targets another creature. The Spell Sniper feat doubles the range of a spell that has an attack role. Changing the range of these spells to Self prevents them from being used with these Feats. The changes to the weapon cost for booming blade and green-flame blade also meant that they couldn't be used with improvised weapons.
These Dungeons & Dragons spells were incredibly useful to have in situations where the party has been captured, as even a stick could be enchanted and turned into a deadly weapon. The fact that the weapons must cost at least one silver piece means that the players will need to find an actual weapon, and not just enchant anything they get to hand that can potentially cause damage. The original versions of these spells and their loopholes weren't game-breaking by any means, and the updates are simply meant to clarify the intended uses for these Dungeons & Dragons spells.