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I want my Doppelganger villain in my D&D 5e game to be a worthy match for my level 7 party, but since it's CR 3, if they get it into combat, though it will be fun, it won't last long.

My goal is to improve my monster so that the monsteriness and the classiness of it are well-integrated: so it doesn't feel to my players as if their antagonist is a generic rogue NPC, but a weird extra dimensional being, but also that they can't mow it down immediately. (Of course, roleplaying is a big part of this, but I'm consulting about rules and stats specifically). I'd be all for homebrew rules.

Is there a good way to add a few character levels to my Doppelganger-- say four Rogue levels-- to fix this problem? I think that should boost its power appropriately, but will it actually do so?

Rubiksmoose
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lowercasename
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    What makes a way "good" instead of "bad"? Doing this in "a good way" seems to be an important part of answering your question well because you say it twice, but it's not clear what you actually mean by it. Are you actually asking about workable homebrew rules for this? Or are you asking about how to integrate class features and monster features? Or something else entirely? Put another way: you could just go ahead and do it without us, so what badness are you hoping we can help you avoid? – SevenSidedDie Jan 03 '15 at 02:50
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    So you want the NPC to feel rogue-like, but also clearly be a doppelganger, and not just some rogue with the Disguise Self spell? Can you edit those constraints into the question? – Oblivious Sage Jan 03 '15 at 03:39

1 Answers1

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The DMG has rules for this on page 283 under "Monsters with Classes". You should probably read them, but essentially you add the class levels you want the monster to have, giving it the full benefit of those class levels, then recalculate its Challenge Rating based on its new stats, using "Step 4. Final Challenge Rating" on page 274. Its proficiency bonus is based on its final challenge rating.

As for whether this will boost your Doppelganger's power appropriately, it depends. The Doppelganger is strongest when deceiving players and attacking them by surprise. 4 Rogue levels, especially if you choose the Assassin archetype, will make him a lot better at that. So if he attacks a single player by suprise, he may well be able to 1-hit KO them. On the other hand, if he fights the whole party at once without surprise, he might never land a hit.

Depending on how you're hoping to have the fight play out, you may want to consider shoring up his weaknesses rather than boosting his strengths even further. You said "if they get it into combat", which suggests to me that you're more interested in making it capable of fighting the party head-on than making it better at sneaking around and killing people in the night. If this is the case, 4 levels of Fighter or even Barbarian would probably go further towards toughening it up than 4 Rogue levels.

Lexaire
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Miniman
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    In addition to upping the lone doppelgänger's CR the way Miniman suggests, try to tie in some other enemies that would fit into what/whoever the doppelgänger is impersonating. So, a fake crime lord would have mercs/thugs, fake politician might have a hired assassin or two. Perhaps a 'family' of three that all turn out to be doppelgängers. Whatever works with the story you're telling. You make a harder fight and perhaps only have to add an ability or two that lasts as long as the doppelgänger is in that form. – Korack Jan 03 '15 at 05:31
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    I'm not sure if you're allowed to use comments for anecdotes, but thanks to you guys, my Fighter 4 Doppelganger destroyed my party and now two players are now rolling up new characters... :D – lowercasename Jan 04 '15 at 09:27
  • @lowercasename I'm hoping everyone had a good time with it! And yeah, CR calculation is tricky and won't necessarily match the actual difficulty of the fight. – Miniman Jan 04 '15 at 10:30
  • I would suggest doing 'test run' battles with your PC's sheets vs the villain to tweak it. Don't involve the players obviously but if you have access to their sheets, you can model how a fight might go and get at least a reasonable estimate of how the PCs can do vs the Doppelganger –  Jan 18 '19 at 16:15