Alright, here's the deal. I'm currently running my first HB campaign and I need my players to start investigating a certain occurrence, but I'm not quite sure how to subtly guide them there without giving away too much.

Now, the basic plot you need to know at the moment is as follows: a group of necromancers is trying to free an ancient banished god. To do so, they are planing to use their necromancy to create a vessel for their god to inhabit. So far however, they failed to create anything sturdy enough to hold the essence of a god, so they have been experimenting in order to improve the process. In addition, they worked with a coven of hags to poison the local guard's water supply with a substance that puts them under a subtle charm that interferes with their ability to think properly and deal with crime, so that the cult is free to go about doing its business without interruption. With the guard taken care of, they started by robbing cemeteries for bodies to use in their experiments, but recently have exhausted that avenue and have gone over to abducting and killing people. It's during one of those attacks, that they first came into contact with the players.

Now, I've been trying to hint at the fact that the guards are under the effect of a spell, but, while my players most certainly noticed the incompetence of the guard, much to their chagrin, all it really did was create a sort of meme of "haha, dumb guards", but not much more.
So the question for me right now is: How can I get my players to start investigating into what's effecting the guard, without outright telling them, "Hey, there's something wrong with the guards" ?

I'd be happy for any tips or ideas any of you had how to deal with the situation and as always, thank you for your time in advance and a wonderful rest of the day to all of you^^

I would think this would be a common thread of discussion among the townfolk. "Garand used to be such a sharp fellow, but lately he moves about like his head is stuffed with cotton."

These guards have families and lives outside of their jobs. I'd imagine their loved ones would have definitely picked that something is wrong and they might even be desperate enough to ask that group of adventurers at the inn for help.

So much depends on what your players are interested in. 

You have a huge, elaborate plot going on, but if all the players want to do is kill stuff and get loot, it all goes to waste.  It is unlikely they will have any desire to investigate anything.  If your players are actually into social stuff and looking around for clues, have them arrested.  Have the guards follow them around, refusing to talk, and observing their every move.  Have the people in the local tavern go on and on about how the guards keep walking into some building for no apparent reason.

Good luck.

Quote from frostfoxfive >>

Alright, here's the deal. I'm currently running my first HB campaign and I need my players to start investigating a certain occurrence, but I'm not quite sure how to subtly guide them there without giving away too much. 

[Jamie Hyneman voice]Well, there's your problem right there.

Players don't do subtle. Not at all. You have to smack them around the head with clues.

There's a general rule of thumb for clues in RPGs - make sure there are THREE opportunities to gather each clue.

Don't worry about being too blunt. 

Mmm... well the reason you want the players to investigate the guards is to lead them onto finding the hags which would lead onto the necromancer cult plot I assume?

if the players have missed this clue they could still stumble onto the activities of the necromancer cult, what with the guards total incompetence being the talk of the taver and the wealthy of the local growing impatient with the lack of results some hefty bounties might be going to find some recently missing wealthy families son/daughter... not to mention a load of paupers sons/daughters going missing but no ones offering a reward so who cares...     

If the towns water source is tainted then you have a hook right there. Everyone needs to drink water presumably. As the enchantment effects the guards only, pick one or more players with a similar alignment to the guards. Have the player directly effected in one degree or another. The charm spell could even go wrong or be corrupted by the hags to attract fated souls (hero's) into their service. Causing the players to deal with the problem directly. When in doubt effect the party directly. They will then have no choice but investigate the cause fully. No doubt tracking the cause to the water supply, which leads to the hags, who oust the cultists or have correspondence in their lair sealing their deal with the cultists. 

So as others have said if they need to suspect the guards incompetence is worth investigation they need to know it’s an anomaly. So it wouldn’t just be the hags and cult, all crime my would be on the rise, give them a side quest with a local crime boss. Let them be able to directly or indirectly find out that the town has become easy pickings

I think that the suggestion of having one of the players affected by the enchanment is the most promising if they metagame and having the townfolk comment on it is the ideal approach, but the best option is probably to outright tell them if they fail to catch on. A wandering alchemist or mage might have noticed what is causing the issue and approach the party as the closest competent unaffected group to investigate or the daughter of a guard might tell them her suspiscions that some sort of spell is over the guards because nobody else believes her.

This substance that's in the guard's water supply, is there any reason the cult would not have put the whole town under this effect? Otherwise the guards might be noticed to be useless. An entire village, town or city of people who are useless is going to be noticed as rather unusual. have the town drunk contact the party to tell them about the problem with the towns water supply. To quote the town drunk in Diablo, when the well is poisoned and you ask him about it - "Heh, you drink water?". The drunk was only unaffected because he only drinks alcohol.

If this is a magical malediction, then perhaps the guards start to build up a resistance or tolerance to it. They might start to become aware of things - perhaps the party sees a guard frantically trying to get their fellow guards to stop staring idly into the trees. If the party goes to help, the original guard will shortly stop trying and start staring idly at the trees. This should tell them something's wrong.

 I need my players to start investigating a certain occurrence

Well, this is your problem right there. You need them to do one specific thing. Unless that is something obvious and something the PCs would be naturally inclined to do, then you can't guarantee they will do it. Even if you were to tell them it's going on, what is their reason for investigating? 

I think the chain of events you have planned rests too much upon a knife's edge... to paraphrase Galadriel from the Fellowship of the Ring movie, stray but a little and the story fails, to the ruin of all. The Necros are teamed with hags to poison a water supply to charm the guards... Why would the players even care this is happening? Do they have a reason? Have you given them any clues that it is happening, or that they might want to stop it? Why would you think, in the first place, that incompetent guards would lead to them examining the water supply, and how will examining the water supply lead them back to the hags? And how would doing that lead them back to the Necros?

The charmed guards are only a hint, and not a very strong one. I'm not sure as a player, it would occur to me to investigate goofy guards either. Especially if this is a homebrew world, and it's all new. Have the players encountered super efficient guards before? If not, then they might just think this is how you, as a DM, RP guards.

As a DM, I prefer to set up "here is what the bad guys are doing, and their plan going forward," and then I describe whatever is going on right now, as affected by the villains' actions, and let the players decide what to do. For example, right now, they are bringing the Roman army with them into the Shrine of the Kuo-Toa to get back some religious articles taken from the Temple of Diana by the fish-people. I did not tell them to do it, and if they had not, the religious articles would have just stayed there. I merely described that the temple was closed, locked, and guarded, and no one would be allowed in, because items had been stolen from it. The party then chose to investigate. If they had not, perhaps the Kuo-Toa would have come back and stolen more treasures, perhaps from the Temple of Neptune this time. Or maybe they would have started killing people. If the players don't act, the villains' plans advance. Sooner or later, either the villains win, or the PCs interfere.

So... maybe suddenly the guards are fully controlled now and they all start walking, zombie-like, in a line toward the necros' base. The players see them sleep-walking their way out of town, unresponsive to anything the PCs say, and abandoning their posts. This is happening because the charm has worked, and no one has stopped it. Now the PCs know that something strange is going on and will start investigating.

Here's the thing. Players are always going to miss clues. So you have to make sure there are multiple clues. Do they really have to investigate the guard? What if your players drink the water and then you have them start rolling Int checks at disadvantage. (Secretly roll a Con check for them behind the screen. Only those who fail are affected.)