The most well-known DM activity is the event, also referred to as a plot or quest. It involves any situation where the DM takes control of a narrative without denying players the ability to influence its outcome.
There are two types of Event. One is spontaneous, the other is scheduled. A spontaneous Event can grow out of an Ambient encounter or the activities of a group of PCs. The DM observes an opportunity and uses it. That does not mean no planning or preparation goes into the Event, but it takes place without prior announcement. If it's easy to find enough Players online, spontaneous Events work well. Scheduled events are announced in advance. Players find it helpful if you give them a brief description of what will happen, when it will happen (date, time, timezone, duration) and where so they can be in the right place at the right time. Characters may need some form of motivation so it helps if you have some idea of who you're running the event for. You can also provide a level range in advance so Players know which characters to bring and which to leave at home, as well as a maximum party size.
Most DMs can manage around 3-5 Players at once. If more Players respond or show up on the day, you may need to recruit help from team members (and consider splitting the party) or turn Players away to ensure everyone gets the attention and time in the spotlight they deserve. Sometimes you'll want specific PCs to be present. It can be a nightmare to coordinate everyone and find a suitable time, so you're often better off running two or three separate Events for smaller groups instead. Don't try to please everyone at once.
Every Event starts with an idea, whether that comes from a book you read or film you saw, from the dreams of a PC or any other source. One interesting approach is to encourage players to contribute Event ideas, not for their own PCs but those of others. If an Event is particularly large or influential, you may want to discuss its impact with the rest of the DM team so everyone has a voice in where the server's going. If it's more humble, talking about it can still be a great way of refining your intentions.
Your idea will probably grow into a few 'chapters.' The first chapter brings the group together with a purpose. The following chapters see the group tackle a number of challenges on their way to the climax, all of which offer some clue to what's going on but few of which are absolutely essential. After the hardest obstacle is overcome, the last chapter deals with rewards and consequences. This is, in most cases, the preferred structure. If you threaten to run out of time, you can safely remove a few chapters. Just as importantly, if PCs take an unexpected direction you can change the order of the chapters readily. The trick to flexible DMing is simply to keep a library of NPCs, story fragments and puzzles you can insert into almost any situation to buy time to prepare the next area or lead the PCs back onto familiar ground.
Because atmosphere is an important part of events, using pre-written material for conversations and descriptions is very helpful. You won't have to think up metaphors on the spot, you won't have to type anything out, and barring a few tweaks to fit the situation, you can simply copy/paste text directly from notepad. To create a good atmosphere, use creative writing tips for your room descriptions, tweak fog colours, express the urgency of a task repeatedly and show not just what an NPC says, but also how they seem to feel about their situation. If it's a detective-type quest, pummel the party with information. If it's horror, play up the psychological aspects and make sure they never quite know what is coming for them.
On the day of the Event, make sure to arrive early so you can decorate terrain, customize monsters, check your descriptions or create last-minute items. Start when you said you'd start, and if anyone shows up late use your best judgement on whether to work them into the adventure or leave them out.
An Event cannot exist without some kind of challenge, be it a puzzle, monster, trap, negotiation or moral dilemma. Without challenge, there is nothing to accomplish or lose and therefore no opportunity for a PC to grow. Depending on the length and complexity of the Event, you may want to string multiple challenges together into a single narrative. Often, the nature of these challenges should vary so that Players with different preferences all have something they like, while PCs with different strengths all have a chance to shine. Knowing your Players and PCs can help you select the right ones.
To deal with challenges, PCs need to be able to make meaningful choices. That depends in large part on the information you give the Players. Choosing left or right with no other knowledge is meaningless. An open choice (There's a daemon on the loose, what do you do?) appeals to some but leaves others craving direction. It's often better to suggest 2-4 approaches (fight it, research it, evacuate the village, call in the paladins). If they choose option 5 instead, so much the better.
The final part of a challenge is the outcome. This must be seen to be a result of the choices the party made. You can make this easier on yourself by avoiding pointless rolls of the dice; any time you introduce a random element, you allow for either success or failure. If you can't happily accept either outcome, don't ask them to roll. Combat, of course, is the ultimate random element. Even a lowly level 1 goblin can destroy a level 8 party in theory. Don't start a fight unless you have a plan for what happens when one or more PCs (are about to) die. In addition, it helps to keep your mind open to creative alternatives. You can make a puzzle with only one solution, but if the PCs don't get it the game bogs down. It's (usually) not your job to say 'That doesn't work,' rather, give them a DC and decide what unintended consequences flow from their actions. In showing the outcome of an entire Event, highlight both the good and bad consequences of the PCs' choices. Be kind, because you want to validate their efforts and allow for poor choices made because of miscommunication on your part, but don't be shy about piling on hard-earned guilt.
There are four disruptions you and your Players may have to deal with when you run an Event, so it is good to decide a policy for them in advance.
The first of these is Players logging off before the end. It leaves them with an unfinished story, the others with a weaker party and is usually a result of poor time management. Always keep an eye on the clock. We spoke earlier of shortening chapters. If that's not possible, try to realize on time that you need to steer the story towards a natural break (the villain heard the party coming and slipped away an hour ago, but left encrypted documents behind which may reveal where they went). That way, you can pick up the tale next time without trapping the group in a time bubble, which leaves PCs unplayable and assumes you can successfully get the same group of PCs together again in the near future. Managing Event duration minimizes the amount of players who have to log off early, and ending before anyone has to log off gives the PCs the opportunity to roleplay the aftermath.
The second disturbance comes in the form of afks and crashes. It happens sometimes, but if it happens often it will drain the pace out of any adventure. You might rule that if a single Player is afk, you'll wait 2 minutes and then continue on without them. Or that there's a 10 minute break after every 50 minutes of play and other afks will be ignored. Find something that works for you and your players.
The third mood killer is OOC talk. Some of it will come from players who ask for clarification, some of it comes from you as you explain what's going on or instruct the group to make rolls. This is of course entirely justified. The rest comes from the party as they trade banter amongst themselves. You'll have to decide for yourself if that's making the game more enjoyable or destroying the mood in any specific situation. Simply request they tone it down as needed.
The fourth disruption is one of speed. Players who type quickly can easily overshadow those who take longer. Players who move quickly can be out of a room and into combat before you've time to spring your ambush or request a roll. And some battles happen so quickly that no one's sure what's going on. Don't be afraid to use the Pause button when you need to slow things down for your own sake, to give the group time to react to a PC with negative hitpoints or to stop a bad fight from getting worse by giving the PCs a choice of flight or surrender. You can even agree upon a sign (/dm !!!) players can send to request a pause of their own. Other players outside your event will survive that disruption.
Your role as event DM is to narrate for and challenge the players, not to defeat them, not to shelter them. Sometimes you'll make mistakes. You can own up to them, find an IC excuse to explain what happened and compensate the characters as necessary. Other times the PCs make the mistake. You should give them a chance to salvage the situation, but you don't need to hand out free resurrections.
At the end of an adventure, or sometimes just before the final showdown, it's good to reward the PCs. Rewards can take the form of sums of coin or other treasure, magic items, grateful NPC contacts and so on, but the traditional one remains XP. Depending on your event and the PC's contributions, something in the 25-100 range is appropriate. A PC should never walk away with more than one new magic item and you should not feel compelled to give the whole party magic items either (there'll be other events in the future). Coin and treasure should be in line with normal values for the party's level.
After you've rewarded the PCs, all that's left is to use the forums to provide a brief summary (3 sentences) of the event. List the PCs involved, anything particularly memorable they've done and how they've been rewarded. You can also suggest ways to modify the module so that the PCs' actions can be seen to have affected the world. Informing the team like this is important because it allows us to determine which PCs need more attention and which ones can wait a while before their next big reward.