The sun here is a living creature, a "solar beast" that rises with a crown of fire and destroys everything false in its path. Lies shrivel, crinkle into smoke, and are burned down to their roots. Even the shadows, which you might expect to be the lies' hiding places, are left stranded and dried out. Nothing deceptive can keep its dark shape once the light arrives.
The speaker seems awestruck and fiercely satisfied rather than afraid. The verbs are violent (burn, bust, roiling, plasters), yet the effect is cleansing, as if the brutal force of the light is a kind of justice. The mood is triumphant and intense, almost like a chant or a ritual. That feeling peaks in the closing refrain, "radiance let loose," repeated three times, which works like a drumbeat or a shout of release. The poem never says what the lies are, which lets the sun stand for any truth too bright to resist.