Guides can be tricky.
On the one hand, sometimes the characters need some nudging and assistance. Waterdeep: Dragon Heist seems to be to be rather prone to this – if they don’t find the next link in the chain, or they fail the skill check to recognise it, they will need some sort of guide to point the way to the next stage. (Caveat: I haven’t actually played it, just read it through.)
On the other hand, you don’t want a guide doing everything for the characters or being too much more powerful. There certainly seems a risk of this with the guides in Tomb of Annihilation, and this will definitely be a problem if they are allowed to take up with Artus Cimber, as is suggested – 15th level (based on his hit dice), CR7 and with the Ring of Winter, and the group would take up with him on their hexcrawl search for Omu at levels 2-6. Any foe which would make a decent challenge for the group could be taken out by Artus on his own. And on top of that he comes with his sidekick Dragonbait.
So how do you strike a balance?
Well, the first option is to make the path more obvious in the first place, and less fragile to links breaking due to unfortunate rolls of the dice. Or have multiple paths, so if one avenue fails, they have an alternative, which is maybe less optimal or costs more, but still allows the adventure to go forward. The Angry GM talks a bit about this near the bottom of his post about the gap between Adventure and Encounter, and I’m sure there was another relevant blog post by someone I read recently, but I can’t find it just at the moment.
Another option is for the guide to be a non-combatant, so it’s clear they won’t be helping out with battles. They could be a mentor, a goal doner/gold owner who hangs at home giving objectives and whose door is always open. Clifton Caldwell became such a character in my first campaign – owner of Caldwell’s Cauldrons, the local general store (and, it gradually emerged, as part of a wider network of shops and trading), whose deliveries were being hit by the bandits raiding the trail from the south. He had a good knowledge of people who could be called on or asked for information and developed into a mentor. He also provided occasional additional (level-appropriate) muscle. But he always stayed in town himself.
It’s when your guide becomes a tag-along NPC (as SlyFlourish describes) that things get tricky. There’s another character to worry about during combat – and do you run them as DM (along with all the foes) or pass them out to one of the players to run, in which case they may be tempted to use the NPC as cannon fodder. That’s if you can get around the power imbalance. When you come to a negotiation, the guide may be the most qualified to handle the negotiation, but then as DM, you’re just talking to yourself and removing player agency. Or the NPC might not say anything…in which case they just feel like dead weight or people forget they are there.
I’ve had a few tag-along NPCs during the last couple of years. Conn, the one which worked best I added because I thought it would make an interesting storyline to have a sleeper werewolf in the party, who was only recently infected and had no clue himself. So somehow he ended up not taking damage from attacks which looked like they should have hit, but otherwise, he seemed like a normal fighter. It was going to be full moon along the road as they were travelling south, but before we could get that far we transitioned into Castle Amber, and the situation in the basement exposed him. So that fizzled out 🙁 Also, the two additional fighters provided by Clifton Caldwell were clearly just to help out in battle and show the way to his place in Verge, and their participation was limited to the single decoy caravan journey.
But even these fell into the “well, are they going to contribute to the conversation” problem. In the end I resorted to them generally agreeing with the party, or occasionally pointing out something I wanted them to see, or nudging the party the right way, but unless I had a reason for them to nudge, it always felt awkward knowing whether they should say anything, and if so, what they should say.
I think I’m going to try to avoid tag-along guides in future. Once we finish Castle Amber, we will return to Akorros where most of the current batch of PCs originate and I can develop a network of local guides who they can bump into or search out without the it being relevant or appropriate for the guides to come with them.