Unfortunately, requiring the capture of adventurers is extremely problematic in the middle of a game of Dungeons & Dragons. It is something that needs to be very carefully handled.
The problem derives from the nature of D&D. The essence of the game is that you present challenges to the players, who then discover ways to engage with them: either by defeating them or by avoiding them in some manner. Player agency: the ability of the players to impact the story. When you remove that agency – and especially by forcing the capture of the adventurers – it’s something that can create some unhappy players, which is not what you want!
Honestly, there’s only one forced capture scenario that I really like: when the adventurers begin the game captured. When it’s part of the set-up condition of the campaign, then everyone knows ahead of time what the starting point is and can plan accordingly. This is the technique that Out of the Abyss uses, and it’s a good one. If a player doesn’t like the scenario, at least they know going into the campaign what the start is – it isn’t a surprise to them mid-way through.
The unforced capturing of characters is also not a problem; the situation that happens as an organic part of play. You might have a dangerous foe in the adventure, and the players either choose to surrender to it or it captures them… but they could have avoided it or had the possibility of defeating it. That’s fine. You aren’t forcing the capture of the adventurers; it’s just something that happened in the game. At that point, you can continue with the adventurers as prisoners and see what happens next.
However, the situation that causes all sorts of problems is when the story requires the characters to be captured mid-way through. An early example of this can be seen at the end of A3: Assault on the Aerie of the Slave Lords, where the players end the adventure being captured and thrown into the dungeons. The last adventure, A4: In the Dungeons of the Slave Lords is a classic where the players need to escape with very limited resources, but the end of A3 can be uneasy. A much worse example can be found in the Avatar Trilogy (Shadowdale, Tantras and Waterdeep), which showed us all the worst ways of removing player agency.
The recent D&D Adventurers League adventure Shackles of Blood also ran into this problem. Mid-way through, the adventure would really, really like the characters to be captured. And it does so by sending a very dangerous force against the players and allows the players to fight them. And, because it’s a structured, organised play adventure, if the characters get away it causes all types of problems for the DM to get the plot back on track. The adventure does try to offer suggestions as to how it proceeds afterwards, but it’s difficult for the DM and players. When I’ve run Shackles of Blood, my solution was to explain to the adventurers during the game that allowing themselves to get captured will allow them to track the perpetrators to their lair. It’s a solution; it may not be the best one.
The “adventurers are captured by an overwhelming force” situation is the most problematic of all the ways of forcing capture. It’s also the most common. Adventurers are really good at getting out of those situations though, and the heavy-handed DM soon discovers that he or she has to find another way to capture the adventurers – and will often end up with very unhappy players.
If you find yourself designing a scenario that absolutely requires the adventurers to be captured, and it needs to happen mid-way through, you have a problem. At this point, my preferred solution to this situation is to not have an encounter where the adventurers fight against the capture at all. Instead, I would begin the session with “Your foes have captured you. You are now in their dungeon”, and let the players come up with an explanation as to how it happened.
Although what I really prefer to do is to think about the scenario and work out how it could proceed if the characters are not captured, and then design the adventure accordingly. But, as you can imagine, that is a lot of work.
So, if you need to capture the adventurers, think about how it’s going to play. It’s one of the toughest scenarios to pull off successfully.