The current edition of Dungeons & Dragons is well-balanced compared to the older versions of the game, which is why it was surprising one spell turned out to be so overpowered that the developers had to release a rules update to stop it from being abused by players.
The ability to summon monsters is one of the most fearsome talents possessed by spellcasters in Dungeons & Dragons. With a single gesture, a cleric or wizard can summon a towering earth elemental or pack of demonic wolves to their side. Some of the most broken tactics in Dungeons & Dragons involved summon spells. In the current edition of Dungeons & Dragons, there are several Conjure spells which allow players to summon elementals, animals, fairies, or celestials to the battlefield. The character classes in the Player’s Handbook that can cast Conjure Woodland Beings are druids (starting at level 7), rangers (starting at level 13), and bards (starting level 10, if they select it with their Magical Secrets class ability). A single-use of Conjure Woodland Beings allows the player to summon eight pixies to the battlefield and they stick around for at least an hour, which is where all of the problems begin.
The people who are unfamiliar with Dungeons & Dragons might balk at the idea of pixies being a threat, and a cursory look at their stats might support that belief. A pixie only has a single hit point and an Armor Class score of 15, which means they should be really easy to defeat. A closer look at the pixies stats reveals the truth, however, as they can cast Polymorph once per day. Polymorph is one of the most fearsome spells in Dungeons & Dragons, as any enemy is a single failed will save away from being turned into a chicken. Pixies can also turn themselves invisible at will. This means that the player can cast Conjure Woodland Beasts, summon eight pixies, and have them follow their character while invisible and flying, in preparation for attack. With a single spell, druids became the most overpowered class in Dungeons & Dragons.
Why D&D Pixie Armies Are So Powerful
The reason why a group of pixies is so scary is that they all take their action on the same turn. An enemy monster could be on the receiving end of eight polymorph spells in a single round, which means that they will almost certainly blow at least one of their saving throws and be transformed into a chicken or fish. This is less of a problem when the party is facing a group of enemies, but a single powerful foe can be defeated in just one turn with this action. Even terrifying monsters with Lair Actions can end up burning through all of their special defenses when facing the pixie assault, leaving them open to a beating from the players.
This makes pixies one of the deadliest monsters in Dungeons & Dragons. The mass-pixie assault had the potential to be one of the most deadly tactics in the game and a druid could do it once per day when they hit level 7. By comparison, the other spellcasters with the ability to cast Polymorph can only do it once per day at level 7 (except for sorcerers if they are willing to blow through their sorcery points to use it twice), while the druid can do it eight times through Conjure Woodland Beasts.
One of the benefits of releasing an edition of Dungeons & Dragons in an age where everyone has access to the Internet is that the developers can quickly release errata for the rules and declare them to be the new status quo. This is exactly what happened with the Conjure Woodland Beasts spell, as a response in the Sage’s Advice column on the official Dungeons & Dragons website confirmed that the DM chooses which creatures arrive when the spell is cast. As such, they can ensure no pixes show up when Conjure Woodland Beasts is used, and the player is left surrounded by blink dogs instead of pixies. This severely weakened the druid class in Dungeons & Dragons. Had this error cropped up during the days of Advanced Dungeons & Dragons or third edition, then druids would have taken over the lands of Faerun, Krynn, and Oerth with the help of their mighty polymorphing pixie rangers.
Source: Wizards Of The Coast