With Alien: Isolation on our minds, a game where a xenomorph chases you throughout the entire experience, we recall other games with persistent enemies that won't cut you any slack.
Nemesis – Resident Evil 3: Nemesis
Nemesis is such a persistent enemy that Capcom put his name in the title. He, in many ways, defines Resident Evil 3's total experience as he appears frequently to try and kill you with a rocket launcher. He and Alien: Isolation's antagonist would probably get along really well.
Dahaka – Prince of Persia: Warrior Within
Even the weakest of the Sands of Time trilogy had some memorable mechanics and sequences. Dahaka shows up all the time to remind you that you toyed with fate too much, and things must be set straight. And it's all set to a soundtrack by Godsmack. The game's tone may have been at odds with the rest of the series, but encountering the recurring Dahaka lead to some exciting chase sequences.
Songbird – BioShock Infinite
Elizabeth's keeper and protector in Columbia represents an intimidating, and surprisingly sympathetic force. You never get a chance to fight Songbird in the traditional sense, but when he interrupts your adventure, you know he means business.
The creeping insecurity that you made the wrong decision – The Walking Dead
It's not an enemy in the traditional sense. You don't see it, or watch its life meter whittle away as you fill it with bullets, but it's always there, and it's always making you ask yourself, "Should I delete my save and restart?" Something tells me as The Walking Dead moves forward with new seasons, this enemy will always be prodding you, forcing you take notice every time somebody, "remembers that."
SA-X – Metroid Fusion
The last entirely new 2D Metroid title to release (depending on whether you consider Metroid: Zero Mission new) featured perhaps the most persistent of Samus' many enemies that refuse to leave her alone. Between Ridley, Metroids, Dark Samus, and the Space Pirates, Samus is no stranger to people who don't like her and refuse to give up, but SA-X was the most intimidating. With an arsenal comparable to Samus' own collection of weapons, whenever SA-X showed up, you knew you were in a fight, and you also knew it probably wasn't the last time you would meet even if you were able to get out alive.