Last year, WB Games studio Monolith had an unexpected hit with the release of Middle-earth: Shadow of Mordor. It's a superb game overall, but it was the game's Nemesis system, and its ability to facilitate player-driven story and narrative, that really made the game standout from the typical triple-A relase.

Michael de Plater, design director on the game, gave a quick run-down of the system at the DICE Summit in Las Vegas.

"How can we make narrative out of the gameplay?" is the question at the core of the Nemesis system.

One way to find an answer to that question was to identify player "needs." de Plater and his team used psychological theories as reference points when looking to satisfy player needs through Shadow of Mordor.

One applied theory was the self-determination theory, which says human beings have three fundamental needs: Competence (they need to feel effective in dealing with environment); Autonomy (they need to control the course of their lives); Relatedness (they need to have relationships with others). Shadow of Mordor and the design of the Nemesis system sought to satisfy those three elements.

de Plater also said Player Experience of Need Satisfaction and GNS Theory (which attempts to explain how RPGs work) were also highly instructive in the making of the Nemesis system.

By breaking down human needs and identifying psychological motivation, the team was able to get more clarity than they would've if they just chased the amorphous, subjective idea of "fun," de Plater said.

Designing a system that enabled player-led stories also meant that the game's designers needed to put aside their own desire to tell a grand story, and instead focus on ways players can create and share their own experiences, de Plater said, an approach that is at odds with more traditional storytelling in games.

Narrative design had to evolve. Part of the challenge for designers was determining how much information and story elements they ought to give players, and how much should they let players fill in the narrative blank-spaces. It was a careful balance to strike, de Plater said.

In order to hit that balance, Monolith turned to established, existing systems. "One of our big references in designing this was sports," he said. "Sports are designed as systems that generate stories every year."

From the draft to the playoffs to the Super Bowl, for example, sports offer the framework for storytelling, and then it's up to the athletes, the fans, and other parties that fill in the blanks. That's what Monolith wanted to do; provide the framework and avoid getting in the way of player-led, emergent storytelling.

The team used a few specific tactics to facilitate emergent storytelling. For example, the use of "memory" was implemented as a storytelling technique. Shadow of Mordor would "remember" your actions, and point them out to you in the narrative, putting you and what you've been up to at the center of the story, over and over again. (de Plater pointed to the Psycho Mantis encounter in Metal Gear Solid as one example in which a game "remembers" what you're doing -- an encounter that thrilled players.)

Revenge was also a motivational design tactic -- Shadow of Mordor used player death not as an end state, but instead presented player death as an opportunity to avenge oneself.

Relationships with villains were a key part of the system as well. Constant power struggles -- basically turning Mordor into a giant prison yard -- helped drive stories.

de Plater said the game designers who want player-led stories need to understand that they're facilitators of improvisation. "Every time a player does something, never shut them off," he said. "Build upon [what they did]. … We can take that and build a story from it."

Also make sure to check out de Plater's Shadow of Mordor postmortem on Gamasutra.