I already talked about this in brief right here, so if you're looking for context check out the original feats thread.
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Anyways, onto what we're here for. Warlocks and other spellcasters in 4th Edition have the ability to do stuff with stars, mostly the former since some warlocks make pacts with them. I'll be taking a look at two particular abilities here. We'll also be assuming this takes place within the Forgotten Realms since that has its own cosmology that we can use later, even though this feat applies mainly to those within the Nentir Vale setting.
Star Pact Warlocks get their powers from making pacts with Elder Evil stars from the Far Realm. For the sake of the calc (and the ease of my sanity), we're going to assume that, since much of the Far Realm has been invading the Prime Material Plane, that these stars are roughly the same distance from Toril as Proxima Centauri, the closest star to our own, is to Earth.
We're also going to assume this takes place during the course of 1 round, which in D&D translates to 6 seconds.
Distance from Patron Star to Toril = 40,208,000,000,000 km
40,208,000,000,000 km = 40,208,000,000,000,000 m/s
40,208,000,000,000,000 m/299792458 = 134,119,451.39727297 = 134,119,451.39c
40,208,000,000,000,000 m/6 = 6,701,333,333,333,333.33 m
6,701,333,333,333,333.33 m/299792458 = 22,353,241.899545c
That's a pretty big number. To even things out, we're going to do this again, this time assuming the star is right at the edge of Realmspace, the solar system that Toril resides in. Doesn't make sense at all with the context of the setting, but I feel like we could use a low end just so this doesn't look completely baffling.
Realmspace reaches out to 5,100,000,000 km so that's easy to work with.
5,100,000,000 km = 5,100,000,000,000 m/s
5,100,000,000,000 m/299792458 = 17,011.768855 = 17,011.76c
5,100,000,000,000 m/6 = 850,000,000,000
850,000,000,000 m/299792458 = 2,835.2948091842c
Results
Warlock uses Dire Radiance = 2,835.29 (low-end)
Warlock uses Dire Radiance = 22,353,241.89c (high-end)
Since this is an ability gained at Level 1, alongside many other abilities like it, pretty much everyone within the Nentir Vale (alongside other settings like Eberron and Forgotten Realms) should scale to this. Especially considering this particular edition gave everyone and their mother interrupts: offensive or defensive abilities that can be used in response to an attack. So there's no reason to assume that characters can't react or even dodge this.
Dungeons & Dragons Feats and Lore General
In Baldur’s Gate: Descent into Avernus 5e campaign book, you have a Dragonborn Paladin of Tiamat by the name of Arkhan the Cruel from Toril via Forgotten Realms setting who had a brief visit to the world of Exandria (the same world where Critical Role and The Legend of Vox Machina takes place)...

Anyways, onto what we're here for. Warlocks and other spellcasters in 4th Edition have the ability to do stuff with stars, mostly the former since some warlocks make pacts with them. I'll be taking a look at two particular abilities here. We'll also be assuming this takes place within the Forgotten Realms since that has its own cosmology that we can use later, even though this feat applies mainly to those within the Nentir Vale setting.

Star Pact Warlocks get their powers from making pacts with Elder Evil stars from the Far Realm. For the sake of the calc (and the ease of my sanity), we're going to assume that, since much of the Far Realm has been invading the Prime Material Plane, that these stars are roughly the same distance from Toril as Proxima Centauri, the closest star to our own, is to Earth.
We're also going to assume this takes place during the course of 1 round, which in D&D translates to 6 seconds.
Distance from Patron Star to Toril = 40,208,000,000,000 km
40,208,000,000,000 km = 40,208,000,000,000,000 m/s
40,208,000,000,000,000 m/6 = 6,701,333,333,333,333.33 m
6,701,333,333,333,333.33 m/299792458 = 22,353,241.899545c
That's a pretty big number. To even things out, we're going to do this again, this time assuming the star is right at the edge of Realmspace, the solar system that Toril resides in. Doesn't make sense at all with the context of the setting, but I feel like we could use a low end just so this doesn't look completely baffling.
Realmspace reaches out to 5,100,000,000 km so that's easy to work with.
5,100,000,000 km = 5,100,000,000,000 m/s
5,100,000,000,000 m/6 = 850,000,000,000
850,000,000,000 m/299792458 = 2,835.2948091842c
Results
Warlock uses Dire Radiance = 2,835.29 (low-end)
Warlock uses Dire Radiance = 22,353,241.89c (high-end)
Since this is an ability gained at Level 1, alongside many other abilities like it, pretty much everyone within the Nentir Vale (alongside other settings like Eberron and Forgotten Realms) should scale to this. Especially considering this particular edition gave everyone and their mother interrupts: offensive or defensive abilities that can be used in response to an attack. So there's no reason to assume that characters can't react or even dodge this.