How to reliably yet reasonably kidnap a PC without making it feel unfair?

I know it sounds iffy, but I know my players won’t mind (and in fact will probably love it). Here’s the short & sweet context:

  • Lesser antagonist, a changeling, is on a mission to steal the lamp that our genie warlock’s efreet was trapped in, to give to his boss. His boss is the one who had originally trapped the efreet in the lamp, and wants its power. Boss lives in another city. Neither of them knew that the warlock PC freed the efreet, who is now his patron.
  • The party encountered this changeling a while back and thought they killed him…but they left right after K.O.ing him without double-checking, and I rolled 3 successful death saves (no one even asked me what I was rolling!) so he’s still alive, taken on a new identity now. Party has met him in this new identity and doesn’t suspect a thing. 
  • Recently there was a bit of a happy accident where the warlock PC openly admitted to the disguised changeling that he’s got an efreet patron who’s been giving him power. This changeling now knows that stealing the lamp is pointless.
  • So now, the only obvious solution? Steal the warlock instead. 

Boss lives in a city that’s a 3-day boat ride away; the party knows he’s there, and have been planning to go confront/kill him (since the efreet wants vengeance for being trapped) once they were strong enough. That’s our next arc. But now I’m thinking it would be perfect if the changeling kicks things off by kidnapping the warlock PC at the end of a combat, stuffs him in a shipping crate to send to his boss, and the rest of the party has to rescue him. (He won’t be able to pull this off alone, but I can definitely give him minions/allies.) 

Addressing potential concerns y’all might have: 

  • I’m certain the warlock’s player will absolutely love the drama of this.
  • Ideally, I’d like to find a way for the party to save the warlock PC before he’s shipped off, so that he’s not separated and kept out of the action for too long (maybe like, half a session??).
  • My players have told me they don’t mind some light railroading for the sake of the narrative.

Here’s what I’m struggling with:

  • The party consists of 5 level 8 PCs, and they’ve gotten pretty dang strong…the warlock is a powerhouse in combat. Subduing him is going to be genuinely difficult.
  • The player would go along with it and let the warlock be kidnapped if I told her about it, but I don’t want to spoil the surprise/plot twist!
  • Even if I do successfully subdue & grab the warlock without telling the player first, between our glory paladin, evoker wizard, mastermind rogue, and beastmaster ranger/moon druid, I’m genuinely worried about my odds to successfully steal away with him without them being able to grab him right back.
  • So basically, I need a 99% foolproof way of this ending with a kidnapping…but I don’t want to press some sort of overpowered anti-fail button that makes them feel totally powerless and cheated.

Any suggestions to pull this off? Spells, magic items, strategies, situational setups, etc.?

EDIT: Bunch of great suggestions and points already made! Lots of people saying I should 1. talk to the player, and/or 2. leave a fair chance for the party to successfully foil the kidnapping attempt. And you guys are right. Thanks for pointing that out!! I'll make sure to do at least one of those two things. Will definitely be mulling over the ideas y'all have shared!