Ability to match backstory - magic item

[DarcyFinn]

Playing Curse of Strahd (heavily home-brewed), and the party has now reached level 5. The Paladin of vengeance has spent time attuning to a axe he found and has discovered that there is more to it than first met his eyes (he has had it for about 5 game nights). I have informed him of the following -

It appears to have been originally made in a time before Strahd by Dragonborn smiths in lands far from Barovia.
You are able to determine that it was forged as part of the great dragon war against the 2nd rising of Tiamat - God Queen of the Chromatic dragons, and is imbued with a spark of metallic dragon essence in their fight against the great evil.
You do not know how it came to be the family blade of the D'eath's but you can only imagine that they once did a great service for one of the mighty Dragonborn clans and that original handle was later replaced with the ornate rose and windmill design. However you now notice the curling pattern on the blade 'which you thought to be thorns' is in fact a stylised rendition of dragon fire which must have been part of a now obscured original design.
You feel that with more study you might be able to determine more about the nature of the spark within the blade and the power it holds.
-

The blade has randomly (on a DM roll) cast green flame blade a couple of times, but this has been highly situational and more to indicate the blade has more to it than he might have though. However at level 5, and after some attunement I would like to give the Axe a permanent ability.

I tend to throw quite hard creatures at the group, so was wondering if a 'on long rest' charged effect and a flat damage bonus would be OTT. I was going to let him determine the dragon spark inside the blade and 'boringly' (but the player loves rolling dice) just let the blade have a extra 1d6 or 1d8 worth of relevant elemental damage.

TLDR
Can anyone suggest some good weapon 'on long rest 'charged abilities that would go with a Paladin oV /dragon theme?
What flat damage roll would you add to such a blade?

[Unoriginal]

Once per long rest, it casts the spell Dragon Breath on the wielder?

[nickl_2000]

Pretty much any level 1 or 2 fire spell would be appropiate.

Also a fear effect would be exceedingly appropiate.

Or a short term fly effect.

Also absorb elements gives the idea of dragonic resistance and more dice rolled once per day. In fact this one may for perfectly, and will remain useful all through his career.

[DarcyFinn]

Thanks for the suggestions. Would you say a level 2 version of absorb elements would be too big a buff?

[Unoriginal]

Thanks for the suggestions. Would you say a level 2 version of absorb elements would be too big a buff?

Magic items don't have to follow spells. I'm sure there are weapons or other magic items in the DMG that already have a protection from element effect you could take inspiration.

[saucerhead]

The suggestions of Fear and Absorb Elements are good ones, but if you think either is too much, you could give it a drawback. The wielder is hated by animals, or is greedy or miserly. Not combat effects, just fluff.
Although, if a familiar swoops at him in combat...