Compared to Holy Avenger, it needs some toning down
First point: your weapon as statted is a Longsword. There is no need to create a 'bastard sword' and offer the heavy property unless you want to penalize small creatures who may want to wield it. If you want to do that, go ahead and do it. That won't impact the 'balance' much.
Second point: you have made an artifact level item, not a legendary one.
Holy Avenger is a Legendary Sword, which has these features.
Weapon (any sword), legendary (requires attunement by a paladin)
You gain a +3 bonus to attack and damage rolls made with this magic
weapon. When you hit a fiend or an undead with it, that creature takes
an extra 2d10 radiant damage.
While you hold the drawn sword, it creates an aura in a 10-foot radius
around you. You and all creatures friendly to you in the aura have
advantage on saving throws against spells and other magical effects.
If you have 17 or more levels in the paladin class, the radius of the
aura increases to 30 feet. (SRD, p. 225, same as DMG)
Step 1: Adjustment into 5e mode
Dawnblade, Magic Longsword, +2 Rarity: Legendary
Properties: Heavy
This magical Longsword is known as the Dawnblade.
As a Bonus Action, you may speak Dawnblade’s command word, wreathing the blade with a radiant flame. Whenever in this state, the blade emits bright light in a 60 foot radius with, and an additional 60 foot radius of dim light. {Note: this the typical radius pairing for light implementation in D&D 5e; see torches (20/20), wall of light (120/120), daylight (60/60) light cantrip (20/20), etc}.
Undead, Fiends, and Evil Fey within range of the bright light have disadvantage on their attack rolls unless they cannot see.
The bright light activates Sunlight Sensitivity for creatures that have it, unless they have Blindsense or Blindsight. {the -3 to perception is needless complication, so I removed it: disadvantage on attacks is a major protective benefit}
Additionally, all Undead and Fiends within the range of the Bright Light take Radiant damage equal to twice your Intelligence, Wisdom, or Charisma modifier at the start of their turn, ignoring resistances but not immunities. {Using Ability score as the damage basis is way out of scale for this edition of the game; using an ability modifier is the more typical model}
A Cleric, Warlock, or Paladin of 16th level or higher use four times their Intelligence, Wisdom, or Charisma modifier, while a Fighter of 18th level or higher uses three times their Intelligence, Wisdom, or Charisma modifier.
{This is kind of fiddly, and most 5e weapons don't add this kind of extra detail, but some do have class based benefits so we'll roll with it).
Your Weapon Attacks with this weapon deal an additional 2d8 of radiant damage. This damage changes to 2d10 radiant damage when the weapon is wielded in two hands.
A Paladin attuned to this weapon can choose to change their Divine Smite damage to Fire Damage. {Since radiant damage is rarely resisted, and fire damage is frequently resisted, this isn't much of a change and it's cool flavor}
Step 2: Scaling it to Holy Avenger
But wait! That's far more powerful than the Holy Avenger. Larger area of effect, and an additional damage rider beyond the 2d8 or 2d10 radiant damage. The area of effect damage really boosts the power although it's only +2 to hit and damage.
While the saving throw boost is quite powerful on Holy Avenger as a defensive measure, inflicting disadvantage on attacks (1/400 chance for a crit versus 1/20, and a +3 to +5 effective boost to ally defenses depending on how high their armor class is) is also a great party protection feature. Probably a wash unless all you encounter is spell casters. (If that's the case, Holy Avenger is a lot better since more saving throws will come up). Triggering Sunlight Sensitivity is a nice defensive feature although it overlaps quite a bit with the disadvantage for some creatures.
As another point of comparison: if you look at the Sword of Sharpness (very rare) it requires a natural 20 to trigger additional damage. Always on AoE damage is quite powerful. I scaled it down to use the ability mofidifer as the basis. If you want to keep Ability score as the basis, with those multiples, you are well into artifact category.
To illustrate what I mean: If my Figher has a Charisma of 10 at level 18, and you keep the multipliers, he does 30 radiant damage in a 60' radius. That's between 5d10 (27) and 6d10 (33) on average area damage per turn in a 60' radius.
I will still recommend that you make this an artifact in its current (somewhat edited form).
If it's an artifact: give it beneficial and detrimental properties.
Artifacts are not well balanced and they usually have side effects. Your attributes are fine for an uber-weapon. A detrimental property or two will help it better fit into the artifact category. Here's a thematic suggestion:
- For an artifact beneficial property "the sword's bright light is sunlight"
is a nice thematic addition to its other properties and makes it
even more lethal to any creature that is harmed by sunlight.
- Pick the detrimental property / side effect with care. I like the detrimental property that induces a roll on the DMG insanity table; it is both flavorful and somewhat unpredictable. Here is what I did with that for one of the feature pairs for an artifact I created (with an initial review by @NautArch and a few others).
- At level 13, the wearer gains the “Fortitude of Unlife” property: if damage reduces the wearer to 0 hit points they can make a Constitution saving throw - DC = ½ of the damage taken unless the damage is radiant or fire damage. On a success, the wearer drops to 1 hit point instead.
- Major Side Effect: Madness; short term effects. Each day at dawn, while attuned to this item, roll on the DMG table (d100). This effect cannot be removed while attuned to the item.
This is purely a matter of taste, but if you look in the DMG artifact creation tables, adding a detrimental property, or two, makes this super-powerful sword have some drawbacks. Their power and impact are all over the map. You'll need to find the detrimental property / side effect that feels right to you. Select one (I suggest against rolling randomly, you want to keep this thematic with your hand crafted item) or use one on the table as an inspiration for you own unique detrimental property that you slightly modify to keep the thematics consistent.
About Sunlight Sensitivity: you need to call that something else.
Sunlight Sensitivity is a characteristic of some monsters in the Monster Manual that only triggers in bright light, and for a few PC races. It appears that you want to hinder the wielder and their allies, many of whom will not have that feature (unless the PC is a Drow, for example). On the one hand, you can classify that as a Major Detrimental effect (see my point on Artifacts) and Call It Something Else. Sunlight Sensitivity has an explicit in game definition as a racial feature. And if you do that, Artifact is once again where your homebrew weapon fits into the general magic item power slots.
And be warned: that feature will, in all likelihood, annoy the heck out of most of the other players. "Wait, what, I get disadvantage on all of my attacks? Turn that off you fool!" is a predictable response.
For the rare version that you have in mind...
Notice that I do not give this sword a + to hit. Being magical, it bypasses all resistance and immunities anyway. This is on the strong side of rare.
Dawnblade, Magic Longsword
Rarity: Rare
Versatile (1d10), Heavy
This magical Bastard Sword is known as the Dawnblade.
As a Bonus Action, you may speak Dawnblade’s command word, wreathing the blade with a radiant flame. Whenever in this state, the blade emits bright light in a 30 foot radius with, and an additional 30 feet of dim light.
- Undead, Fiends, and Evil Fey within range of the bright light have disadvantage on their attack rolls unless they cannot see.
- Additionally, all Undead and Fiends within the range of the Bright Light take Radiant damage equal to your Intelligence, Wisdom, or Charisma modifier at the start of their turn.
This may still be on the powerful side for a rare item. However, it hits less often - stock rare weapons are +2 to hit, stock uncommon weapons are +1 to hit - so I gave it two features.
For the very rare version
Take the rare version, add 1d8/1d10 radiant damage on a hit, and also add
- A Paladin attuned to this weapon can choose to change their Divine Smite damage to Fire Damage. {This can probably be removed, since adding another feature like this probably puts the sword closer to Very Rare than rare}.
That's close enough for the very rare power range, which has a wide variability.
