Getting a magic sword from a sidequest, what should its effects be?

[Protato]

In a recent game, my Bard Erik's magic Rapier Lunarian was damaged beyond repair, and his brother Albrecht has another Rapier called Solarian. As you can probably guess, they have a celestial naming scheme, and that's related to their powers. Lunarian is a silver sword that glows with the light of the moon, and does 1d4 Radiant damage. Solarian doesn't have any abilities, but it was meant to level with the character and Albrecht's a lower level. I was originally planning to have them use their swords the entire game with no replacement, but my DM has said that the two swords can be fused into one sword (let's call it Galaxias) with incredible power.

This is a good setup, and honestly I find it to be quite creative. However, my problem is not knowing what sort of powers Galaxias should have. I think it should have some sort of glowing and additional damage should be present, but so should other effects. Since my character's a Bard, magic isn't a big deal for him, so the sword being able to cast a spell would be superfluous. I'm also having a hard time coming up with something flavorful, given the celestial theme. If anyone knows something that could help with this, I'd appreciate it very much.

[DevilMcam]

From the theme It could be a sword with either both aspect of the sun and the moon, or changing according to the time of the day.
Even if you are already a spellcaster having spell imbued into object is good because it let you have some you don't already know.
There are three obvious spell fitting to the theme of sun and moon : Daylight, Moonbeam and sunbeam (but this one is a 6th level, so possibly too strong)

I don't really like the name galaxias but that's purely personnal and flavor so let's stick to it for now
one option could be :

assuming your original swords did not have bonus to accuracy we will give one to the fusion, if they already had one just pump it up
Galaxias + 1 :
weapon (Rapier), Legendary (require attunement)
This weapon emit dim light to 30 ft when unsheathed.
Additionnaly when imbued in moonlight or sunlight (be it magical or natural) you deal extra damages based on the light : 2d4 radiant damages when striking in moonlight, 1D4 fire +1D4 Radiant when striking in sunlight.
Once per day (recharge at dawn) you can as an action use the sword to cast daylight centered on the sword, if cast this way the duration is reduced to 10 minutes.
Once per day (recharge at dusk) you can as an action use the sword to cast Moonbeam as a level 2 spell without needing material components. You don't need to maintain concentration for the spell as the sword does it for you, however you still need to use your action if you wish to moove the beam.

This would be a very powerfull magic Item IMO, make sure to discuss with your DM about it.
And anyway, make sure you Discuss with your DM about this item, even if you don't take any of the suggestions that will apear here.

[Talamare]

Galaxias

This weapon destroys all light within a 5 ft radius while unsheathed, treat this area as natural darkness.

This weapon has no physical presence and can never do physical damage. On hit a target takes 1d10 Force Damage. Then must make a Constitution Save and a Wisdom Save. On failed Constitution Save the target suffers 1d10 Radiant Damage. On a failed Wisdom Save the target takes 1d10 Psychic Damage and becomes Frightened. The DC for the Constitution and Wisdom Save is equal to 10 + The amount of Force Damage taken from this hit.

A shapechanger makes its Constitution saving throw with disadvantage. If it fails, it also instantly reverts to its original form and can’t assume a different form until a short rest.

[Hopeless]

What level is this Bard?

Off the top of my head I'd say a +1 weapon with the ability to grant it's wielder +1 caster level to any spell they can cast themselves as long as they're wielding it whilst doing so?

[Protato]

What level is this Bard?

Ah, my apologies for not saying his level. He's a level 12 Lore Bard and Level 2 GOOlock, I was planning him to be pure bard but it made sense for him to be GOOlock for story reasons (long story).

[Specter]

- +1 to hit and damage
- Whenever the wielder of the sword scores a critical hit with it, the target of the attack is also subjected to a magic missile spell (cast as a 6th-level spell).

Just a thought.

[Slipperychicken]

In a recent game, my Bard Erik's magic Rapier Lunarian was damaged beyond repair, and his brother Albrecht has another Rapier called Solarian. As you can probably guess, they have a celestial naming scheme, and that's related to their powers. Lunarian is a silver sword that glows with the light of the moon, and does 1d4 Radiant damage. Solarian doesn't have any abilities, but it was meant to level with the character and Albrecht's a lower level. I was originally planning to have them use their swords the entire game with no replacement, but my DM has said that the two swords can be fused into one sword (let's call it Galaxias) with incredible power.

This is a good setup, and honestly I find it to be quite creative. However, my problem is not knowing what sort of powers Galaxias should have. I think it should have some sort of glowing and additional damage should be present, but so should other effects. Since my character's a Bard, magic isn't a big deal for him, so the sword being able to cast a spell would be superfluous. I'm also having a hard time coming up with something flavorful, given the celestial theme. If anyone knows something that could help with this, I'd appreciate it very much.

Start with the effects of a Sun Blade: +2 tohit and damage, +1d8 radiant, and the ability to shine light out to 15ft. Flavor it as having that "it's full of stars" look. The stars shift as it moves. If held up to a starry sky, you realize that you're actually seeing the celestial bodies behind the blade, as if the blade "sees through" the atmosphere into the stars.

I'd add to that a "Black Hole" power that makes enemies make a save or be pulled closer to the user and damaged. Make the player shout a command phrase like "GALAXY POWER: BLACK HOLE, ACTIVATE!" before it can be used. Also a "Big Bang" power that pushes enemies away while damaging them. Obviously you'd need to make the player say "GALAXY POWER: BIG BANG, ACTIVATE!"