I have just read this post, and I have a different take on the idea.
I love the narrative possibility of replicating the moment of Bard's Destiny; but I think the Black Arrow idea should be tailored to be quite a unique event.
In the Hobbit, the Black Arrow was not really such a special missile.
[/color] The real effect of the Black Arrow was because Bard hit the one place where Smaug was not covered by his armour. ALL arrows were incapable of Wounding him (even the Black Arrow I would argue) if they hit his diamond-studded skin.
IMO having a super powerful Arrow is the wrong solution. Instead......
SPOILER ALERT the following paragraph is using very dark text:
There is a similar idea of Destiny in Tales of the Lone Lands, with the story of Estelang. Where that is more useful, is that the sword is not Used Up in a single attack like the narrative for a single powerful arrow would be, of course!
SPOILER ALERT ENDS normal text returns ....
What follows is long (sorry for that) but it breaks down my reasoning into
A. The Bowman and the Bow: not the Arrow
B. The Arrow
A. If you do not want to waste several Valour upgrades on a feature you can use only once per combat at best - or even that you might lose altogether if you miss - it is better to reward the Archer or their Bow.
Rationale: Bard was already a powerful archer. He would certainly have one of the better weapons available from the Laketown arsenal. So I would expect him to have (say) a Superior Grievous and a Superior Fell weapon already. It may be such a well built weapon it can have an extra Opening Volley if not surprised/ambushed. After all I imagine that it would be a great advantage for Laketown's defenders to be able to cut down the Bridge to the shore, and yet still have bows capable of reaching any foes stuck on the shore. As a Watchman in the guards, this matches Bard's condition as I imagine it. (Maybe Dour handed?)
So the PC who is destined to hold a Black Arrow of their own would be a developed archer already. Naturally these abilities will be available for ANY normal archery attack the PC would make, and would not be lost after firing any Black Arrow: so this avoids that huge plot risk/cost.
B. So, the PC already has the capability to take down ordinary Adversaries, even moderately powerful ones. But the wicked LM springs a truly stupendous foe on the party, the narrative rises to a crescendo of doom and agony.....
'Black arrow. Black arrow. I have saved you until the last.... .... go now and speed well!'
In such a moment of absolute crisis, it's time for a super powerful effort. Naturally the PC uses their great bow again. With their great archery skill and their determination (high Valour) they can easily strike the mighty beast confronting them: but this Foe needs more than power and skill to be defeated, more than ordinary bravery to stand up against and to select the right moment to fire. It is a time of last resorts - of last arrows...
Black Arrow Rule: when the PC rolls a Hit in combat, they can THEN elect to declare that it was their last arrow, their Black Arrow. (Note: Your PC believes that if this does not work, further archery will be worthless against this foe, and they will cast their bow aside for the rest of this combat!)
Having hit the foe, the PC can use any Additional Success Icons (tengwars) on the d6 Success Dice to select and apply any COMBINATION of the following effects. An effect can be selected more than once.
(i) the numeral on the Feat dice is increased by 3 (or the Archers Valour rating) making a Wound more likely (like a better Pierce option)
(ii) the Armour roll used by the foe is reduced by 1d6
(iii) the number of Wounds is increased by one
Note: the archer must not use their bow again in this combat (last arrow)
Note: option (iii) can only be used if a Piercing Blow has occurred already or by applying option (i) it has been created.
Note: I have no opinion about recovering the Black Arrow
Example: using everything they have got a Valour 4 archer with 5 in archery skill, spends Hope and rolls 4 tengwars with a Feat die score of 6. This results in a Hit and the PC declares they are using their Black Arrow. After this attack, they are out of arrows.
They are facing a Dragon with Might 3. An earlier hit by a dwarf with an Axe has Wounded this beast once, but it fights on with 2 Wounds remaining.
Your PC chooses the following....
Spend a tengwar on (i) to apply the better Pierce option and gain a Wound/Piercing Shot
Spend two more tengwars to reduce the Dragons Armour rating from 5d to 3d
Spend the final tengwar to add another Wound roll: this would also be at 3d Armour of course.
.... is it enough?