There's an enemy far greater than any end-game boss in the lives of many MMO players. It lurks in the lairs sinister and foul, takes possession of things both great and small, and smites with blind contempt on both the worthy and unworthy alike. It's the enemy that causes even the tamest of players to unleash a string of curses, the most dedicated to ragequit, and many to simply spend hours in the same place with their forehead adhered to their desk. It is the three-letter beast: RNG.
Yes, the RNG – or random number generator – is a terrible beast. Ultimately, RNG determines all our combat actions (with rare exception, particularly with action MMOs). The roll of invisible, virtual dice determines whether we hit or miss, whether our attacks will be deflected or resisted, and of course, whether or not we'll get the loot we most desire. Random number generators ensure that we have some cap on our personal skill, that we must balance our stats appropriately to account for chance as best as possible, to take control out of the player's hands. It also serves as one of the most frustrating elements of any game.
The element of the RNG began in the very roots of modern RPG gameplay: table-top RPGs (aka pen-and-paper). These games relied on rolls of the dice for just about everything, and if you've ever heard of things like a “natural 20,” this is the source. A good dice roll is typically high in numbers, and players often lamented of “cursed dice” or those which, when it mattered, never seemed to roll high enough to suit their needs. Critical rolls were those at the extreme ends, and the bane and blessing (depending on the nature of the roll) of any player.
RNG eventually found its way into many single player games, most often to add challenge to an otherwise straightforward game. For example, look at Dragon Warrior 2 on the NES. There was a spell called Sacrifice (Kerplunk in newer versions of the series) that one of the characters could learn, which sacrificed the caster in order to defeat all enemies. Its use was situational at best, but useful in certain scenarios. Now, give the ability to an enemy on the road leading up to the final boss, with no ability to save in between starting the area and finishing it. Many a player has lamented seeing those Gold Batboons, watching them cast Sacrifice to kill your entire party (no resists here to save one's hide) and set you back to the last save you made, minus half your gold pieces. All in the name of making a game more challenging.
RNG has developed over time from the unfair incarnations in earlier games. In an attempt to make MMOs more “realistic,” many developers coded RNG based systems into the NPC AI, the combat systems, even the loot systems themselves. This is why we end up with instances where a mage decides to melee someone instead of casting spells, or why that precious item we want never, ever seems to drop for us. This is a system where in a raid, everything can be going perfectly, and lost instantly with a sudden streak of rotten RNG strings – where even the best players can end in a night of frustration and defeat because the system rolled against them.
So why use RNG elements in the first place? The simple reason is they take control out of the player's hands. While there are certainly many formulaic calculations done for most modern MMOs, if there was no element of randomness, it would be quite easy to figure out what to do when. Many older games were built on this foundation, and a pattern system was the result. Once you learned the boss patterns, spawn timers, and appropriate counters, the game suddenly became a walk in the park, losing its challenge and appeal.
Game designers now seem to be steering away from this situation as much as possible. That isn't to say that they have stopped the use of RNG entirely; it makes many pieces of the immense MMO puzzle easier to process. Nor are patterns necessarily wrong, even in an MMO; cyclical boss battles can still be challenging. A careful balance has to be maintained between random actions and predictable ones; a simple “If boss does this, group reacts by doing that” encounter becomes simple and boring; a complex battle that randomly spits out death bolts can break the best players and discourage them from trying a battle further.
A good boss battle, or difficult encounter, should have multiple complex elements to it. Players enjoy being challenged and overcoming issues of skill, coordination, reflexes, and teamwork. Good elements of encounters can include things like berserk timers (time limits that force both a skill and gear check on players), effect avoidance (dodging spells and abilities requiring situational awareness), and reactive skill use (changing abilities, skills, and tasks throughout the encounter as the encounter requires). Bad elements include things like insta-death abilities (death beam, really game?), forced incapacitation that fails to take into account a group's composition or needs at the given moment, and elements that put into conflict issues of time versus latency that make a slightly under-performing computer or uncontrollable latency situation a liability.
RNG elements can fall into good or bad categories. A properly designed RNG element can test teamwork, response time, and flexibility of the group members. A poorly designed RNG based ability can slaughter a raid through a chain of uncontrollable events. Each game's design will differ, as will its difficult encounters, but whatever the case, RNG must be used with consideration for – dare I say it? - player's feelings. Players simply don't like being screwed after they did everything flawlessly because of a roll of the dice.