Stat blocks are one of the most complex parts of the game, but also the most useful. They tell you everything you need to know about a creature or character’s abilities in a fight, much like a condensed version of a character sheet.
How you use stat blocks is up to you. Some Game Masters like to create custom stat blocks for most of the allies and enemies encountered during an adventure, some like to create them only for the biggest and baddest enemy characters, and others are perfectly happy to repurpose statistics from other adventures or books like the Alien Archive. Some GMs don’t even bother with full stat blocks and just write down a few key statistics—Armor Classes, attacks and damage, Hit Points, and saving throws—and ignore the rest unless it becomes important.
All of these approaches are valid, but in general, the ways you expect your party to interact with a character determine what you need. If your PCs go to a nonplayer character (NPC) for research assistance before their next mission, then you probably need to know only a few skill values, whereas you’ll probably need to know all the combat statistics for the Free Captain pirate they battle in the adventure’s climax. Also remember that in addition to using published characters and creatures as written, you can simply “reskin” those creatures. If you use the statistics for a haan but describe fins and jets instead of claws and balloons, a cold spray instead of firespray, and a swim speed instead of a fly speed, congratulations—you’ve created a brand-new alien, and your players will never know the difference!
Level Equivalent for Monsters and NPCs
Many abilities and effects are based on a creature’s level. Unlike player characters, however, monsters and NPCs don’t have levels. Instead, the CR of a monster or NPC functions as its level for any ability or effect based on level.