I want my party of two nps and 3 pcs, all level 10, to fight an npc of mine (a sorcerer) but I don't know what level to make him in order to give them a hard-level encounter.
closed as too broad by Purple Monkey, daze413, Miniman, okeefe, Oblivious Sage Apr 26 '17 at 1:39
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The info for how to create encounters with classed NPCs is all contained in the DMG, but it is a bit spread out, and not all in the "Creating a Combat Encounter" section on page 81. A good starting point would be "Creating a Monster", p.273-283, it will walk you through the process of designing your NPC.
Also, it should be noted that character level might not be the biggest contributor to the CR of your encounter. Obviously it allows for access to higher-level spells, but you could easily make a low-level NPC quite dangerous by outfitting them with magical items which negate or counteract the abilities of your PCs. The notes in the DMG are helpful but won't necessarily account for the individual strengths and foibles of your party!