I generally follow a few simple steps.
Step 1: during the session 0 I let them know that there may be encounters they cannot win.
Step 2: set up an encounter they can't win.
Step 3: Broadcast, broadcast, broadcast. Make it obvious they cannot win.
Step 4: Give them multiple routes of escape. If I want to make escape something more difficult than "we bravely run away", allow for failures but figure out some kind of cost for failure. If they fail horribly and repeatedly go to step 5.
Step 5: Kill them all.
Of course there is always the alternative to step 5 of playing "capture the PC". Maybe the ogres want to pay a special tribute to their queen and the group has a chance to overhear some tidbits about the planned invasion or there's someone paying a premium for living subjects which explains all the mysterious disappearances of late. The players should have some input into this decision, some would rather just start up new PCs.
Step 1: during the session 0 I let them know that there may be encounters they cannot win.
Step 2: set up an encounter they can't win.
Step 3: Broadcast, broadcast, broadcast. Make it obvious they cannot win.
Step 4: Give them multiple routes of escape. If I want to make escape something more difficult than "we bravely run away", allow for failures but figure out some kind of cost for failure. If they fail horribly and repeatedly go to step 5.
Step 5: Kill them all.
Of course there is always the alternative to step 5 of playing "capture the PC". Maybe the ogres want to pay a special tribute to their queen and the group has a chance to overhear some tidbits about the planned invasion or there's someone paying a premium for living subjects which explains all the mysterious disappearances of late. The players should have some input into this decision, some would rather just start up new PCs.