In many forms of table-top role-playing games there’s an unspoken contract between the DM and players. The player-characters will defeat (or sometimes fail to defeat) or overcome (or sometimes fail to overcome) a nemesis. Even when the players are playing evil characters, there are nemeses to defeat, whether those nemeses also be evil characters or whether they be the do-gooder paladin types.
When it comes down to a battle, players are not well-known for showing mercy to their opponents, especially if they have reason to dislike the opponents, perhaps because he’s beaten the players before, perhaps because he’s taunted and mocked them, perhaps because of something in the back story. Whatever it is, it’s unlikely (well, at least for the players that I know and have roleplayed with) that when their nemesis is at their mercy, they’ll let him go.
What then, when the nemesis needs to get away, so that he can do the ‘So, we meet again, Mr Bond’ type encounter?
Well first, what not to do:
DM: As you start to draw your knife across his throat, the mage reaches into a pocket, pulls out a wand, mutters a complex spell and vanishes.
The players are likely to feel cheated, and rightly so. Especially if it happens more than once.
If the nemesis needs to get away, plan the encounter around that fact. Plan an exit route into the encounter from the start. Maybe the fight is on top of a sky scraper and a helicopter snatches the bad guy away. Maybe the encounter is next to a fast-flowing river that the ‘defeated’ nemesis can fall into. Maybe there’s a zip line, an elevator down to a getaway car., a contingency spell in place, etc
Whatever route is chosen, the players should feel like they had a chance of victory, if *that one roll* had gone better, it they’d realised the implications of the signal lights, if they’d had someone downstream, etc, etc. Not cheated that the victory was taken from them with nothing that they could do about it.
And if the players do manage a complete victory despite all the preparations. Well then maybe it’s an opportunity for a different scene:
You killed my Master! Prepare to die!