There are some Dungeons & Dragons monsters that players are terrified in meeting in battle, such as the Rust Monsters and their magic item breaking touch, or the Mind Flayer and its paralyzing mental blast. There are some foes that players love to fight, and evil adventuring parties should be on the top of the list, thanks to the challenge and the treasure they provide.
The vast majority of D&D parties are composed of good or neutral-aligned characters. These adventurers are usually motivated by the desire to protect the weak, perform good deeds, and maybe make a little profit on the side. There are also characters who are motivated by more singular desires, such as revenge, but they will still follow their conscience when evil beings appear.
There are some adventuring parties that are motivated by greed, with class builds that serve darker powers. They only join together out of a need for mutual protection, or the knowledge that they can achieve more as a unit than working alone, especially in a world full of crusading Paladins and meddling Wizards. These parties go on their own adventures and complete their own quests, which will eventually put them into conflict with player characters.
Evil D&D Adventuring Parties - The Mirror Match
There are a few reasons why evil adventuring parties are the best villains that a DM can use. For one thing, they provide the best challenge, as they possess similar capabilities and powers to the party. They match the action economy of the players, while they lack the hit point sponge nature of the bigger boss monsters. The group will now have to work out counters for their own tactics and face the wrath of their strongest abilities. They also work as a great counter to the concepts of each character, assuming the DM wants to go that route. A life-worshiping Cleric who faces an evil Necromancer in Dungeons & Dragons has both an ideological foe and someone whose powers perfectly oppose their own.
The other reason why players loved facing evil parties (especially in older editions of the game) is the treasure. An evil party needs to be loaded down with the same types of magic D&D items as the player, in order to put up a fight. Once the battle is over, then the victor takes the spoils, leading to the best hauls short of a dragon's hoard. Evil adventuring parties always have the best loot, and they put up the hardest fight for it. There are lots of enemy types that Dungeons & Dragons players can face, but few are as fun as evil adventuring parties.