How many computer games have you played which started with your character being either dead or in prison? TESO actually managed to start your character being both dead *and* in prison. That sort of game start establishes a motivation for your character: You dislike the people who threw you in prison, while you like the powers that resurrected you from death. That works well at the beginning of a story, because you don't have to mess with player agency to get them into the death and/or imprisoned state. If you want to motivate your players in the middle of a campaign with either revenge or gratefulness, you first need to engineer a situation where outside forces do bad things to the players.

The problem is especially acute in episodic campaigns, where there isn't a strong loyalty of the players to somebody else. Game of Thrones tells strong stories because the characters have strong bonds to their respective houses. In a typical episodic campaign the player characters have little interaction with their family, if they have any at all. You can't just introduce a family member into the story only to use him five minutes later as a leverage, having him threatened or killed only so that the player is motivated.

Revenge can make for great stories, just think of the Count of Monte Cristo. For the player to be really motivated by that revenge, you need to do something to the player character. Which isn't all that easy if you think how D&D tends to be a series of events and encounters where the players emerge victorious pretty much all of the time. Engineering an encounter the players are bound to lose is already messing with player agency. And it tends to be protracted and chaotic. So if you want to suspend player agency and put players into a bad place from which a story of revenge, or a story of rescue and gratefulness can evolve, it is better to do so with a very short event.

In the example of my campaign there deliberately was no warning. The players had set up a guard, who saw a black cloud appear, but couldn't do anything to stop it or warn the others before everybody fell unconscious. There deliberately were no saving throws, or other rolls of the dice, or opportunities to act against the event. The transformation was quick and inevitable. And thus the suspension of player agency was short. The story moved on very quickly to how the players reacted to waking up in a room full of dark gnomes. And from there to the imminent threat of being a group of dark gnomes in a city which is already in a panic about an "Underdark threat". So after the transformation, player agency was quickly restored. The situation had changed fundamentally, but they were back in control of their actions to deal with that situation.

In summary, good stories evolve from the interaction of the players with unforeseen events and outside forces. For that it is sometimes necessary to suspend player agency while these events occur. The best is to keep that suspension short and give the reins back quickly to the players to deal with the new situation.