During battle and some other, rare circumstances, First Ones can be captured alive by their enemies. In most cases, a seriously wounded First One or one surrounded by enemies with no chance of escape, will surrender using an ancient, traditional formula.
You can also surrender voluntarily at any time to any First One within interaction range.
First Ones can only be captured by other First Ones, and once so they will automatically move around with them, with no movement option of their own. Likewise, their available actions are now limited, but not zero. There is still a lot that a prisoner can do, from messaging to many activities. The most important restriction is travel - and everything that comes with it.
A captured First One can be set free, executed or handed over to some other First One. If a First One who has prisoners is killed, those will be transferred to whoever killed them, or if they did not die in battle they will be set free.
These simple mechanics make all kinds of deals, diplomacy and negotiations possible. Both sides, however, need to establish trust first. For example, executing a First One will kill them, but that is it. As a prisoner, you could bring them to one of their estates and force them to hand it over, as payment for their freedom. Of course, you could still kill them afterwards...
Prisoners can be very valuable, but also a burden. The repercussions are manifold, and giving the matter and all circumstances thorough consideration will almost always be more rewarding than picking one rote reaction for all cases.
Some things work differently when prisoners are involved:
Some things work the same, though this might seem unexpected at first:
Prisoners can make attempts to escape, but since they are valuable, the chances are not actually that great. A failed attempt will be reported to their captor, who could decide to let consequences follow.