Next session my players are going to fight a black puddin in a nearby cave. That they already know. What they don't know yet is that inside this cave there'll be a hidden entrance for a secret room. Inside this room, a treasure is kept. A very powerful Djinni, able to grant wishes, kept inside a iron flask. I need a Guardian to place into this room so they can fight the guardian before get access to the iron flask.

My party is level 4, 5 players. (monk, bard, barbarian, rogue, warlock)

Any ideas?

Invisible Stalker, maybe?

A pair of Bone Naga or a spirit naga might be appropriate? Nagas guard things for the gods. It's difficult to balance for a party at this level - you won't find legendary action creatures appropriate to the party's level, and a creature with just one action is going to get taken out pretty easily by a party of five. 

Quote from Farenas >>

Invisible Stalker, maybe?

As per Invisible Stalker description he's more fit to hunt people and stuff and not to guard stuff. But I don't know, other than that looks like a fine choice

Quote from Sanvael >>

A pair of Bone Naga or a spirit naga might be appropriate? Nagas guard things for the gods. It's difficult to balance for a party at this level - you won't find legendary action creatures appropriate to the party's level, and a creature with just one action is going to get taken out pretty easily by a party of five. 

 A spirit naga is too powerful isn't it? But two bone nagas looks nice

Quote from Licozo >>

A spirit naga is too powerful isn't it? But two bone nagas looks nice

I'd be careful; bone nagas based on spirit nagas can cast lightning bolt, which will probably one-shot any of those characters on a failed save (CR here is a total mess; there's no way the cleric spell variant and the wizard spell variant are equal threat).

Quote from Licozo >>

Next session my players are going to fight a black puddin in a nearby cave. That they already know. What they don't know yet is that inside this cave there'll be a hidden entrance for a secret room. Inside this room, a treasure is kept. A very powerful Djinni, able to grant wishes, kept inside a iron flask. I need a Guardian to place into this room so they can fight the guardian before get access to the iron flask.

My party is level 4, 5 players. (monk, bard, barbarian, rogue, warlock)

Any ideas?

Who put the djinn in the flask, and created the defenses to stop someone from releasing it? Figuring that out could help you narrow down what sort of guardian it has (undead, construct, elemental, what have you)

Djinni are intelligent spirits of elemental air, opposed to the forces of elemental earth. A dao or a group of earth elemental myrmidons could be suitable jailers. Alternatively, an evil conjurer wizard who traps extraplanar entities and forces them into service might be cool. Flavor the conjurer as a true namer who learned the true name of the djinn, allowing it to be summoned, bound, and trapped.

There's really two options, the encounter can say something about who's guarding or what's in the vault. 

• If you want to give info about who's guarding it look for something related to who created the vault. For info on that you'd need to tell us who captured the djinn.
• If you want to give info about the [Tooltip Not Found] then use something related to the djinn like an elemental.

It's also a good idea to do more than just combat so here are some suggestions to do a little more with these creatures. 

A suggestion to make it an interesting encounter that gives info about the treasure is to have either an air elemental, invisible stalker or air elemental myrmidon tied to the flask. Then do one or both of the following 

• The elemental is empowered,  or can't be killed without removing the flask from a pedestal or deactivating some magical device on it like smashing a crystal it's encased in.  
• The elemental acts as a barrier when they approach the flask an Air elemental. When a player gets near the flask it appears and uses it's whirl ability possibly knocking them back then because it's a monster they can't move through it's space so it can just stand there and form a barrier preventing them grabbing it until it's destroyed, making melee attacks on those who stay near the flask. This works well as an additive to another monster.

If you do only one of those then it won't significantly increase the difficulty of the encounter if you do both they work together to make increase the difficulty by the cr of the air elemental. That's because the air elemental doesn't pursue players so acts as a trap but if it is protecting another monster through the flask empowering it then it's hp is functionally added to the monsters. 

I would also recommend against giving a party an actual iron flask; that's a potentially campaign-breaking item in tier 4, let alone tier 1.

Quote from Solarsyphon >>

There's really two options, the encounter can say something about who's guarding or what's in the vault. 

• If you want to give info about who's guarding it look for something related to who created the vault. For info on that you'd need to tell us who captured the djinn.
• If you want to give info about the [Tooltip Not Found] then use something related to the djinn like an elemental.

It's also a good idea to do more than just combat so here are some suggestions to do a little more with these creatures. 

A suggestion to make it an interesting encounter that gives info about the treasure is to have either an air elemental, invisible stalker or air elemental myrmidon tied to the flask. Then do one or both of the following 

• The elemental is empowered,  or can't be killed without removing the flask from a pedestal or deactivating some magical device on it like smashing a crystal it's encased in.  
• The elemental acts as a barrier when they approach the flask an Air elemental. When a player gets near the flask it appears and uses it's whirl ability possibly knocking them back then because it's a monster they can't move through it's space so it can just stand there and form a barrier preventing them grabbing it until it's destroyed, making melee attacks on those who stay near the flask. This works well as an additive to another monster.

If you do only one of those then it won't significantly increase the difficulty of the encounter if you do both they work together to make increase the difficulty by the cr of the air elemental. That's because the air elemental doesn't pursue players so acts as a trap but if it is protecting another monster through the flask empowering it then it's hp is functionally added to the monsters. 

I'm now having visions of an air elemental at one end of the room guarding the flask, and an open pit at the other end with some sort of ooze at the bottom...

The story behind to why the djinni is there - even though my players will not care - is that the djinni family trapped him in there, condemned to live on the material plane as "he wished". Or they might just got jealousy of his powers and how he was using them. Liked the idea of the Air elemental as a trap, and for the guardians I was thinking about two boosted animated armors

Now, about the iron flask, do you guys think it's a "campaing breaker"? Yes, they can trap one big boss on it, but they can't trap two, if they want to trap another the first one would be released. So I guess they will use this once.

Or I could just make the Djinni ask (or grab by force) the iron flask from the players, so he can "destroy his prison"

I also intent to grant the players a Wish offered by the Djinni in gratitud for being released.

Quote from Licozo >>

The story behind to why the djinni is there - even though my players will not care - is that the djinni family trapped him in there, condemned to live on the material plane as "he wished". Or they might just got jealousy of his powers and how he was using them. Liked the idea of the Air elemental as a trap, and for the guardians I was thinking about two boosted animated armors

Now, about the iron flask, do you guys think it's a "campaing breaker"? Yes, they can trap one big boss on it, but they can't trap two, if they want to trap another the first one would be released. So I guess they will use this once.

Or I could just make the Djinni ask (or grab by force) the iron flask from the players, so he can "destroy his prison"

I also intent to grant the players a Wish offered by the Djinni in gratitud for being released.

 Wish is OK to give to players as long  as you are reasonable with what you allow them to wish for, try to keep it in line with the power of the things listed in the examples for the spell wish and try to facilitate it through spell effects if you can like a clerics divine intervention. 

For flavour when I do wishes with genies I like to give the wishes an elemental theme because they are elementals but that can be tricky to do if you aren't use to it. As an example for a djinni which is an air elemental they might find the winds guide them to what they seek.

I've never run with the iron flask but it's DC is passable and it only effects extra planar creatures so you can plan around it and it shouldn't break the game if you are careful. 

 Per the xanathar's rules you generally don't want to give out legendary items before level 11 and there really should only be the one at that point.   I personally would not want to give that item out before level approximately 15 as it reminds me allot of the spell dominate monster and that's when a full caster would get that spell.

Yeah I forgot about the part of having control upon the creature for 1 hour. I think I can make a nerfed version of the iron flask then, smaller DC, maybe 15, and it only traps creatures, no control over it.

Flameskulls are fun guardians or a couple Spectators could get the job done.

What would want to keep the Djinn contained?

Maybe an Earth Elemental

You sure you want to be giving the party wishes at this level?

A glyph of warding releasing a living spell such as a living burning hand?