What ways (through buffs or items) can my rogue deliver radiant damage?

Question

I'm a sword-and-crossbow rogue in a party with a wizard, an eldritch knight, a moon druid, and a rather bashy life cleric who didn't take any damage cantrips. Thus we find ourselves a bit short on radiant damage as we head into an undead-heavy campaign.

Now, I know the cleric and druid can prepare radiant attack spells and we can carry holy water, but I'm looking for some more repeatable solutions to allow us (me) to deal just a little radiant damage in order to, for example, halt vampires' fast healing.

To give direction, I'm looking for longer duration buff spells and magic items of any rarity. I am not interested in class abilities or anything requiring multiclassing or respeccing of current characters.

Answer 1

There are a few radiant options open to your party.

1. Moonbeam, a 2nd level Druid spell, is available to the party Druid. It lasts for a minute, concentration, so it's a good way to keep the Radiant damage going without spending a spell slot every turn.

2. Spirit Guardians, a 3rd level Cleric spell, is another spell that deals radiant damage repeatedly if your Cleric can maintain concentration.

3. Look for a Sun Blade for yourself or the Eldritch Knight. It's a rare magic sword with the finesse property that deals Radiant damage.

4. A Mace of Disruption, another rare magic weapon, is also an option. Its only drawback is that it only deals radiant damage to undead and fiends. If you ever need radiant damage against a creature of another type, you'll be out of luck.

5. A Zealot Barbarian (from Xanathar's Guide to Everything) deals additional radiant or necrotic damage on the first attack they hit with on each of their turns starting at level 3. This may not work for your party, as it would require some multiclassing.

6. The Paladin spells (also available to War Domain Clerics) Divine Favor (1st level) and Crusader's Mantle (3rd level) add Radiant damage to your weapon attacks and the weapon attacks of all allies within 30 feet of you, respectively. They each last one minute and require concentration. These are similar to a "lesser Holy Weapon", but may not be easily accessible to your party.

   It's also worth noting that the Daylight spell that you mentioned ironically doesn't create sunlight (see this question), nor does it deal radiant damage. It wouldn't actually be able to help you with a vampire. If the spell did create sunlight, it would say so, as in the 6th level spell Sunbeam.

Answer 2

Magic Items

A Mace of Disruption or a Sun Blade are your best bets. Both are rare items, so they're generally considered acceptable for tier 2 characters. There are other items that come up when searching for "radiant" and "damage" in D&D Beyond, but they're legendary or setting-specific.

The Sun Blade is the best choice for a rogue, as long as you don't mind the fact that walking around with what is effectively a lightsaber makes hiding impossible.

While the blade exists, this magic longsword has the finesse property. If you are proficient with shortswords or longswords, you are proficient with the sun blade.

You gain a +2 bonus to attack and damage rolls made with this weapon, which deals radiant damage instead of slashing damage. When you hit an undead with it, that target takes an extra 1d8 radiant damage.

The finesse property means you can Sneak Attack with it, and because the weapon's base damage is radiant, the entire damage from the sneak attack is radiant, too.

Outside Assistance

Unfortunately, your party doesn't have anybody who can cast Crusader's Mantle. It would be available relatively soon to a War Cleric, or a little bit later to a Paladin. It adds a small amount of radiant damage (1d4) to every successful weapon attack made by a person within 30' of the caster.

Other Options

Unfortunately, there really aren't any in D&D5E. Previous editions had a lot more bits and pieces that have not made an appearance in 5E. That said, I can't recall any that would have helped anyway.