im planning on it, a long quest taking place in 5 different places of the world, a helmite paladin is charged to find 3 keys to a temple to unlock it. a quest start is a quest item describing the quest in general and a place where they can find clues to the 3 keys locations. when they have the quest item will tell them where it is located (by triangulating on a map) - small caveat the continents have moved since the quest item was crafted and therefore is somewhat misleading. then they get to go find it. they are lvl 6 when they got the storyhook - it will be a loooong time (basically they need to be at teleport level due to the distances) before this quest is done.

@Qoray If anything I’d say that’s probably underpowered. Talk to your DM about using a Pike as the base martial weapon. That might work better for what you’re after :) 

I could see the holy avenger template on any weapon, it would depend on the faith. 

I love the idea of the weapon and the wielder developing a bond overtime and as a GM/DM I would make this weapon available to a Paladin as a low level quest reward (level 4 most likely). However, here is what I would do to showcase that growing bond between weapon and wielder overtime; make it scale overtime. 

At Level 3 it would be classified as just a magical weapon that does an additional  2d4 radiant damage against undead. Level 4 it becomes a +1 weapon and at Level 5 it does 2d6 radiant damage to undead. Level 6 it gains the aura bonus and becomes +2. At 7 the radiant gets up'd to 2d8. Level 8 it does the full 2d10 and at Level 10 it becomes the full +3 weapon as shown. 

I'd even give it further buffs though; 

Level 12 it bolsters the Improved Smite (or whatever it's called) feature dealing an additional 1d8 radiant and the weapon also illuminates the area around the wielder equal to the Paladin's aura effect. Level 14 Undead and Fiends within the Paladin's aura have disadvantage on their first attack roll against either the wielder or their allies within the aura. At 17 it crits on a 19. At 18 it crits on an 18. 

Thoughts? 

Quote from UlricKessler >>

-snip-

Thoughts?

 I really like the idea of a paladin's holy weapon growing into a future holy relic and really bonding with it like you mentioned however the idea that the weapon changes like that every single level is a bit "busy" for my tastes. I'd probably go down the route of offering one buff from the Holy Avenger (eg. an additional +1 or the aura bonus, etc.) each time the paladin performs a deed in the name of their deity. This could be bigger events like restoring a temple to their god, desecrating an the shrine of a antagonistic deity or another sort of big heroic deed which brings glory to the deity. This means that the player doesnt need to try and remember a bunch of different changes each time they level, and rewards the player for leaning into the devout heroic archetype and actively pursuing the goals of the deity.

Alternatively, you could let your Paladin "Tithe" powerful magic weapons to their god in order to gain the benefits for their holy weapon. For example, if the paladin gets a +1 shortsword, they can sacrifice it to their deity in order to have the +1 bonus applied to their greatsword instead. If the player tithes a Flame Tongue you could make the bonus 2d6 Radiant damage instead of fire on the paladin's greatsword. The caveat to this would be that the player needs to understand that they're swapping abilities with the existing ability on the sword, not gaining it in addition too. At the point where the player tithes a +3 weapon then you could "max the weapon out" as a Holy Avenger.

Is it just me or is this really underpowered?

Quote from plopped123 >>

Is it just me or is this really underpowered?

 Just you.

So, I was just looking this up, because my DM gave it to me at the end of our campaign. He also used some rules of cool.

We were fighting the BBEG and in our party was a Paladin NPC who had this the whole way through. I had seen it’s power, but had no interest in it, because I had Shatterspike and I prefer niche over power. Things weren’t looking good for the party, a TPK seemed near and  unfortunately the NPC died during combat, but my character who was a level 6 Paladin level 5 Warlock after seeing him die picked it up to avenge his death. My DM immediately said, that you felt the essence of the NPC still within the sword and let me auto attune. Ever so fittingly I rolled two natural twenties for my attacks and the tide of the battle shifted heavily. Morale lifted and we did not TPK. We are about to start another campaign with new characters, but my DM just messaged me saying there might be a nice place for this character to appear and wants to workshop it with me. So, the Holy Avenger may get to see another day.

give it to a party without a paladin

Around the teens probably. But if you wanted to do something neat give them the inert sword as just like a plus 1 sword around level 6 or 7, but tell them they can see that it's power is untapped. Then when narrative allows it have whoever is attuned to it experience a dream that calls them to a side quest that ends up restoring one of the properties of the sword. A plus 2, then maybe an aura of plus 1 on attack rolls, then a plus 3, then the full Aura. Using these missions to act as tension breakers in between arcs or quests in the "main story" 

For this item in old scenarios that had one; Holy Avengers were the base template and were made into artifacts with other powers. That use to be par for the course, not this generic sword. Food for thought for the new players and DMs.

The way I gave this weapon out was by making it a Great sword +1 but the runes on the blade are inert. They have to find a master smith, get a large sum of gold and a unique item to repair it. and they have to repair it in stages. So I am curious to see how this plays out TBH. But I hope this a good system.

This sword would be more interesting if it was the fourth evolution stage of a weapon that leveled alongside your paladin. For example, he starts the game with a greatsword that his father gave him and he treasures it. It gets blessed when dipped into a holy fountain into a weapon +1. A wizard later gratefully enchants it into a weapon +2. When he deals the killing blow to an ancient dragon, its blood infuses the sword, turning it into a weapon +3. Then, when he fulfills his oath as a paladin at a high level in an epic way, it at last upgrades into this.

Quote from EffingRooster >>

As a GM, when would you consider handing out a Holy Avenger and under what circumstances? Of course each campaign is unique and dependencies on magic items shift, but I’m after personal views.

Would you give it out earlier and have it as an item that give the Paladin legitimacy, letting them bond to this renowned weapon and uphold its legacy? Or do you hand this out to a high level player who has fought hard and been true to their oath even when it’s meant following the harder path, finally having this legendary sword deservedly bestowed upon them?

Does their god or order of knights award this to them? Or do they unearth it from a tomb that has been lost for centuries?

Would love to hear your thoughts!

 
Beign a legendary weapon I would say it's at least a 4 step item. 
Obviously you don't have to follow this rule of thumb but when you consider giving "for life" item to a character (it's very unlikely a paladin would ever replace this attunement slot), it's sensible to make the attaining of said item into a tale appropriate to it's rarity. So a legendary item should have a legend to it's aquisition and useage. It really depends on how the champaign is panned or wether there is a follow up. Also weither the adventurers are crafting the item or just looting it.
Either way let's have 4 - 5  milestones: Paladin's Oath, The Fiend, The Celestial, The Great Hurdle. So basically have the character face a trial against their oath. Not all who disagree are evil and not all who agree are good. Have the paladin face the the opponents of his conviction, give their oath substenance and allow the player to show their character's conviction, not just tell. Have the player put their character against the scrutiny of appropriate Celestials. The paladin is not a cleric, and that means there might be a discord between their oath and what the celestials might want. 
And the great hurdle is adventuring besides any holy quests. You can reward with neutral material.
So for instance, your group saves a kindom from the ravages of war or help some dwarves reconquer their lost homeland and be rewarded with a cashe of mithril.
Have the group find a sympathetic cultist or something of the sort. The paladin's actions against this NPC can have a lot of consequence among mortals.
When the group finally  defeat the cultist leader (high level) the Paladin can feel accomplished. As a reward the gods might send a Deva who instructs the group on a highly dangerous but importand follow-up mission. The reward for defeating the high level devil/demon/undead in the end could be some divine blessing (in physical form as well, like a holy chrystal) 
Then maybe they still need to find a smith, but a holy avenger could be made this way. You could even say that when they get the mithril they can forge a sword +1 and when the paladin deepens their oath it turn +2 and when they finish here it becomes the legendary sword.

"As you reach out and touch the holy artifact of Obenruth you feel repelled, the dark shards of your soul seem to be warring with each other. You cannot tell if you feel repulsed by it or a strange sense of yearning for a world where you didn't have to take that path that you did. Nonetheless a deal is a deal, if he will give you back your family no trinket or treasure is to much to pay."

I will always disagree with the damage on rapier and scimitar. To me it seems the Rapier should be  d6, scimitar a d8. But, I'm just contrary.

Critical Role: Season 1 

That is amazing!

Quote from Finn_Blackstone >>

Perhaps even make it the “Excalibur” Blade that grants Hexblade Warlock powers for a just cause, but not wielding it til much later in a multi class situation.

 This is a common misconception that the Hexblade patron is a literal weapon. The Hexblade is a force from the Shadowfell which manifests in many evil sentient weapons created from shadowstuff, such as [item]Blackrazor[\item]. The sentience in Blackrazor is not the Hexblade itself; the Hexblade is the source of the sentient magic in Blackrazor and many other weapons.

Many sages believe the Hexblade is actually the Raven Queen.

What I would do is have their god guide them to it, or have some clerics or high priests of their order tell them tales of the weapon and where it could possibly be. I would never straight up give this item to my players. For an item of this power they have to do at least some amount of work. Plus it should give them a badass story.