A PC is under the effect of a Suggest spell and told to follow someone who had darkvision. The PC, who is under the effect, doesn't have darkvision, and they are led into a tunnel system that is pitch black. How fast are they able to move?
I can't find anything RAW which states that trying to move in darkness has any detrimental effects, other than the obvious statement of not being able to actually see where they are going.
Best Answer
The rules provide no guidance, therefore it is your job as DM to make a ruling.
The obvious upper limit is that it has no effect and you can move at your full movement rate and the obvious lower limit is that it is totally debilitating and you can't move at all.
Did I say no guidance? Well, there is a little bit - crawling, swimming and climbing cost 2 for 1 as does difficult terrain and, of course, if these combine (e.g. crawling in difficult terrain) you are down to 4 for 1.
In making your ruling, ask yourself these questions:
In making your decision try to come to a consensus with your players about what seems reasonable but remember you make the final call. Whatever you decide, stick with it for consistency (unless it is obviously failing).
With respect to the suggestion spell itself:
Walking into the dark might be an "obviously harmful act".