This is what I think the most boring monster in D&D is.
The invisible stalker. It is so close to being cool, and terrifying and a real challenge, but then it just... isn't.
The invisible stalker in D&D lore is a creature summoned from the elemental plane to hunt down a specific object or person. It is described as an unwilling participant in this task and given enough time it may pervert the original tasks into a harmful behavior towards the one who summoned it unless it is worded carefully.
It has two predominantly horroresque features. It is invisible leaving no trace where ever it travels and that it doesn't need to eat, drink, or sleep making it the perfect persistence hunter. This *would* make it a great recurring villain.
A creature that can't be seen, (without select spells), or detected without proper preparation, BUT, it has what I consider to be two massive flaws.
1- The invisible stalker is not a crafty creature. It has an intelligence of 10 (+0), so it as about as clever as average humans in the D&D world. Saying it eventually plans to kill its summoner **should** imply that it might eventually succeed, but considering it would most likely be summoned by a Wizard, whose whole thing is how smart they are, this doesn't exactly present it as a major threat.
2- The invisible stalker only speaks Auran although ot does understand common. This means it cannot convey what it is doing or why to those that it is hunting.
There are a few changes that I would suggest making as a DM to this creature to make it a more credible threat to players.
Change number 1: Change the invisible stalkers intelligence from a 10, to an 11. This has no mechanical benefit, however it does convey to anyone who has access to that information that it is slightly more clever than average, that it might, given enough time think of something to challenge those that summon it, it also implies that the invisible stalker could have the wherewithal to set traps for those that it hunts and can wait for them to go off before it finally attacks.
Change number 2: Either give the stalker, or the players a way to communicate with one another. This will allow the players to begin to try to question or capture the invisible stalker and understand that the thing that is consistently hunting them is actually a lesser threat than the one who summoned it.
Change number 3: Make it so whenever a character summons an invisible stalker it is the same one as previously summoned. This means if your players kill it outright they will still be able to gain some information from it.
From a lore perspective it would also be an interesting idea to make a secret change to the first level spell "Unseen Servant" by giving it a 10% chance to summon an invisible stalker instead of the servant when cast, and behaves as an unseen servant would (while maintaining all the feats and traits of an invisible stalker) until unseen servant is dispelled.
