D&D 5E +1 Shield and Table p. 142-143 of the DMG

[Psikerlord#]

I would probably nerf it a bit to resistance as a reaction once/short rest. And the truth aspect to apply all the time but primarily as a roleplaying aspect - he finds it harder to lie, but he can still lie if he must. Perhaps from time to time if he is trying to deceive someone he might suffer -2 or disad on his deception check.

 

[77IM]

How about: resistant to necrotic damage, but vulnerable to radiant damage.

That sounds like a wash, but is actually a benefit because necrotic damage is much more commonly dealt by monsters and villains than radiant damage. Plus if you encounter some enemy dealing radiant damage you can just take off the shield if it's going to be a problem.

 

[Mhyr]

Yes, I got the feeling resistance to necrotic damage is overpowering the item (I had in mind), even despite the quirk as Psikerlord pointed out.
I fear granting resistance as a reaction / short rest could complicate game play, as some other great ideas.

To balance it with a vulnerability to radiant damage is kind of nice, but you said yourself, 77IM, not very many villains etc. use this type of damage.

At the moment I really consider the suggestion of 3 points "damage reduction" to necrotic damage.

 

[Prism]

And I would love to hear more of your item stories!

Here is a giant slayer battle axe I put in my current Wailing Dwarf based campaign.

Trollthork

Created By: Dwarves
It is durable and has been treated with blueshine which gives it a gleaming deep blue appearance.

History: Heroic
Dwarven runes along the haft tell that it was crafted for Gillian Sorndor of the Kingdom of Ultoksamrin, who was a dwarven ranger and slayer of trolls.

Minor Property: Language
It has the minor property of enabling the bearer to speak and understand giant (the common tongue of trolls) while on their person.

Minor Property: Acidic
When being used against giants and trolls a thin sheen of acid coats the weapon. The additional damage dealt to giants is acidic rather than slashing damage.

Quirk: Aggressive
The bearer feels the urge to slay trolls when in their presence.

All in all this makes it just as effective against giants, but much more against trolls. I also made it into an Urgosh rather than battle axe which I'm treating as a heavy, two-handed 1d10 weapon with a bonus to those with the polearm master feat.

 

[Mhyr]

Yeah! Thanks for sharing!! Your item is quite streamlined to slay trolls with its two minor properties. Perhaps I would add a stronger quirk or intensify the current by adding something like I did with the Spider Staff of my LMoP-campaign:

Quirk: Wicked. When the bearer is presented with an opportunity to act in a selfish or malevolent way, the item heightens the bearer’s urge to do so. (When in doubt the DM may ask for a Wisdom save DC 10)

Although I like the idea to give no mechanical penalty, but a bonus via inspiration if the item is "roleplayed well".

EDIT: The Wailing Dwarf reads ver good! Sounds really interesting!!