I am hoping this is not too broad. I will try to narrow it down to make it acceptable to RPG-SE guidelines.
I am working on a short campaign modeled on the Suicide Squad concept, also used in Escape from New York and to some extent, the Dirty Dozen. The PCs all start as people in prison with a death sentence upon them. The players get to choose the reason, as well as whether they are actually guilty or innocent. They are offered a mission by a black ops civil authority. If they succeed, they are released from their sentence, if they fail...
That's where I need a control. I need a control that will mean almost certain death for them if they fail to complete the mission. I could easily homebrew a magic item or poison, but I'd prefer to work within the established rules. Therefore I need a control that meets the following criteria:
It is an item, substance, ability, spell or otherwise to be found in a published WotC book. I have most of the fifth edition books, and would prefer that edition. An otherwise perfect item from a previous edition would be acceptable.
It must be something not easily subverted. Most poisons can be neutralized with the appropriate spell, a curse can likewise be removed. If it can be countered, it should be only possible with resources available to high level characters. The PCs will be 5th level. So a wish could undo the control, but a Remove Curse or Lesser Restoration should be ineffective.
(a) If possible, it should be something that could be triggered immediately, by a handler. The Suicide Squad wear explosive armbands that can be detonated remotely should they act against the wishes of their commander. This one is not essential, but would be nice.
(b) It needs to have some kind of time limit: 48 hours, 1 week, it doesn't matter. If it cannot have a time limit, then it must meet requirement 3a: be able to be remotely activated. In this way, the time limit is simply enforced by someone at HQ saying "Their time is up. Terminate them."
- It needs to be something that can eventually be safely removed when the task is complete. The government that sent them on this mission is ruthless, but true to their word.
Is there something that meets these requirements?
Note: Since it has come up several times in the answers below, I know that as DM I can create anything I want in order to solve this. I am not looking for any answer that tells me I am the DM and can do anything. I would prefer to do it as close to RAW as possible. If for no other reason, it keeps this question from becoming too broad, and I would not like it shut down when I have already gotten some great suggestions.