In general, if I want the PCs to lose a fight (or surrender), I want them to realize up front that it's not a normal fight, so they don't feel frustrated by losing. Either put them up against enemies who are way out of their league or stack the odds in some other, obvious way.
If the enemy here is a poweful diviner, a ritual could work well. The diviner knows exactly where they'll be staying or camping and sets a ritual in motion days ahead of time. At the moment of the ambush, the magic coalesces and, say, dazes and blinds the entire party for the encounter. Then they have a brief, confusing fight where they can't even see their attackers, and suddenly the ambushers melt away again, taking the artifact and some prisoners with them.
You could even make it a skill challenge, where the party is stumbling around, trying to escape with as much of the party intact as possible. "Success" would be that everyone not fated to be kidnapped comes out of it unwounded, and they get some significant clues about the attackers and where they went.
Either of those (or probably any scenario where the PCs get to try to interfere with the plan) would require some careful planning to make sure things don't feel too heavy-handed during the encounter itself. IMO, being heavy-handed in encounter setup is (more or less) fine, since players expect that to be out of their control. Being heavy-handed during an encounter takes away from the sense that the characters' decisions and actions matter.
If the enemy here is a poweful diviner, a ritual could work well. The diviner knows exactly where they'll be staying or camping and sets a ritual in motion days ahead of time. At the moment of the ambush, the magic coalesces and, say, dazes and blinds the entire party for the encounter. Then they have a brief, confusing fight where they can't even see their attackers, and suddenly the ambushers melt away again, taking the artifact and some prisoners with them.
You could even make it a skill challenge, where the party is stumbling around, trying to escape with as much of the party intact as possible. "Success" would be that everyone not fated to be kidnapped comes out of it unwounded, and they get some significant clues about the attackers and where they went.
Either of those (or probably any scenario where the PCs get to try to interfere with the plan) would require some careful planning to make sure things don't feel too heavy-handed during the encounter itself. IMO, being heavy-handed in encounter setup is (more or less) fine, since players expect that to be out of their control. Being heavy-handed during an encounter takes away from the sense that the characters' decisions and actions matter.