I made a couple homebrew items and I have no idea if they already exist in some form somewhere else, so if they do I would be interested to know them.

Also I am still pretty new to DND in general so if some of these seem over the top good or laughably weak, that might be why.

Dagger of Injection (WIP name)
(Attune) 1d4 Piercing damage.
This dagger has a reservoir built into it that can contain up to 1 ounce of fluid. There is a trigger mechanism in the handle that can be used to release the fluid through a tiny hole at the tip of the blade. It takes 10 minutes to clean and refill the dagger.
Any poison used to fill the reservoir of this dagger will remain viable as if kept in a proper vial or bottle and not expire as if applied to a weapon.
Using the injection feature of this dagger is a free action.

Basically the idea of this dagger is that you can pre-load a poison into the weapon, instead of having to coat a weapon with it as an action or bonus action.

Listening Stone
(Attuned)
After attuning to this stone, you can hear anything it can hear. It can only hear as well as you can.
You gain advantage on Perception (Wisdom) checks that are sound based if you are carrying the stone.
The stone loses it's ability to transfer what it can hear to you if it is further than 1 mile from you.
If the stone is destroyed, you take 4d6 (TBD) Psychic damage and are deafened for 10 minutes.

I think the bonus to perception checks on this might be a little too good, so maybe just a flat bonus like +2 or something instead.

Feedback welcomed :)

I like the dagger. To make it work a bit more like similar 5e things, I might word the injection more like "when you attack with this weapon using the Attack action, you can inject the contents of the reservoir as a part of the same action."

If you wanted to expand its utility, it would be quite useful to also be able to inject potions into allies as a bonus action. I've wanted to make something like this for artificers.

The listening stone sounds a lot like clairvoyance, so you may want to swipe some of that wording. Note that

A creature’s attunement to an item ends ... if the item has been more than 100 feet away for at least 24 hours

So if you want this to be something you can leave somewhere for longer than that, you will need to add some special rules to the item - or if you don't you may want to add a reference to that rule. I think it's a good limitation though. I don't think the perception bonus is too much, many monsters have something similar to reflect their enhanced senses.

If you have a line about it being destroyed, you should probably address what it will take to destroy it. AC/HP and immunities to certain types of damage are usually noted in cases like this. Magic items tend to be pretty durable (and valuable), so I'd probably have enemy creatures take it for their own use instead of destroying it. Lastly, the psychic damage on destruction makes sense but thunder might be a bit more on-theme since that's the type used for sonic damage.

scatterbraind's ideas are pretty good so I'll second them, but here a couple of other thoughts.

The dagger seems okay but could trigger on a successful hit.  And I don't see the need for attunement when the process is so mechanical.

The listening stone is an interesting idea, but there is a contradiction with granting advantage on Perception when it can only hear as well as you can.  The advantage is fine, just re-word the ability to reflect your improved hearing through the listening stone.

Also, it should probably require an action to switch between your hearing and the stone's, like Wizards and their familiars.  Otherwise it might be difficult to distinguish between local and remote sounds.

I think the psychic damage fits, assuming you have established a psychic link with the stone and it is backlash that is hurting you.  You just need to expand on that idea a little more.

Yeah, I think I agree with Gruntler about the dagger, I would consider something like this:

“Fang” or “Snakebite Dagger”*(The hollow blade you mentioned made me think of a venomous sneak’s fangs.)

Weapon (dagger), rare

This dagger has a reservoir built into the handle large enough to hold up to 4 oz of liquid. A trigger mechanism cleverly hidden within this dagger’s grip connects the reservoir to a hollow within the blade itself. You can use an action to transfer a single vial of acid, basic poison, potent poison,* any poison with the Injury type, or up to 4 oz of holy water into this dagger’s reservoir where it will keep indefinitely until either the reservoir is emptied, or the contents have been used.

Whenever you hit with a melee attack using this weapon while the reservoir contains a suitable liquid, you can pull the trigger within the handle as part of the same attack. When you do, the entire contents of the reservoir are pressurized through the hollow blade and out through a tiny hole near the dagger’s tip. With an audible hiss, the pressurized liquid is forcibly injected into the target who must subtract 1d4 from its saving throw against that liquid. If the reservoir contained either acid or holy water, the creature must make a DC 10 Constitution saving throw to resist the effects. If the reservoir contained basic poison, it must repeat its saving throw at the end of each of its turns for the next 9 rounds taking 1d4 poison damage on a failed saving throw.

You can empty this dagger as a bonus action, the contents spray harmlessly to the ground unless you have a suitable container in which that fluid can be collected. After this dagger has been used, or it’s reservoir emptied, the reservoir can only be refilled with the same type of substance until it has been thoroughly cleaned. It takes 10 minutes to clean the dagger, which can be done as part of a short or long rest.

*Potent Poison can be made by characters with the Poisoner feat.

Notes: Damage, Debuff, Combat, Container, Finesse, Light, Thrown (You won't need to include those, the dagger weapon type will add those automatically.)

Dagger of Injection (WIP name)
(Attune) 1d4 Piercing damage.*1
This dagger has a reservoir built into it that can contain up to 1 ounce*2 of fluid. There is a trigger mechanism in the handle that can be used to release the fluid through a tiny hole at the tip of the blade. It takes 10 minutes to clean and refill the dagger.
Any poison*3 used to fill the reservoir of this dagger will remain viable as if kept in a proper vial or bottle and not expire as if applied to a weapon.
Using the injection feature of this dagger is a free action.*4

Basically the idea of this dagger is that you can pre-load a poison into the weapon, instead of having to coat a weapon with it as an action or bonus action.

1. *(That is inherent to the dagger, you don’t need to repeat it.)
2. *(Most things like Poisons and Potions are stored in a vial, which is 4 ounces. You can get familiar with the container sizes here:https://www.dndbeyond.com/equipment?filter-search=container&filter-cost-min=&filter-cost-max=&filter-weight-min=&filter-weight-max=)
3. *(Only Injury type poisons, and the basic poison can work with weapons, the other ¾+ of the poisons would be ineffective. And technically only the basic poison has a time limit I am aware of, the injury poisons all last until used or washed/rained on I believe.)
4. *(There is no “free action” in 5e. things either take a Character’s action, bonus action, or reaction or else there is “no action required,” for example, a character can interact with a single item once on each of its turns (no action required)

I hope that helps.

Edit: “Potent Poison” added, spelling, sentence fragment removed.

I love all the feedback, thanks! The change to the dagger ideas I really like too.