In its original meaning, being a light-bringer is the task of Lucifer, astronomically the morning star that is Venus. This task has later been ascribed to the figure of Christ.
In this tension between Christ and Lucifer, both charged with the same duty, we already sense that to bring the light is a delicate undertaking, as dangerous as it is necessary. Jung ascribes this dual function to the ambivalent Mercurius of the alchemists.
Mercurius as the shining and shimmering planet, appearing like Venus close to the sun in the morning or evening sky, is like her a Lucifer, a light-bringer (ϕωσϕóροζ). He heralds, as the morning star does, only much more directly, the coming of the light. (Carl Jung, The Spirit Mercurius, CW 13, par 273)
Mercurius is by no means the Christian devil—the latter could rather be said to be a “diabolization” of Lucifer or of Mercurius. Mercurius is an adumbration of the primordial light-bringer, who is never himself the light, but a ϕωσϕóρος [phosphoros] who brings the light of nature, the light of the moon and the stars which fades before the new morning light. (ibid., par 300)
Mercurius, that two-faced god, comes as the lumen naturae, the Servator and Salvator, only to those whose reason strives towards the highest light ever received by man, and who do not trust exclusively to the cognitio vespertina. For those who are unmindful of this light, the lumen naturae turns into a perilous ignis fatuus, and the psychopomp into a diabolical seducer. (ibid., par 303)