A guiding light is a magically conjured source of illumination that can be projected through space.
Merlin conjured such a light and projected it into the caves below the Forest of Balor when Arthur Pendragon was left there to die by Nimueh.
When Merlin conjured this light, he said the incantation "Fromum feohgiftum on fæder bearme. Fromum feohgiftum" (The Poisoned Chalice).
Merlin conjures a sphere of light on his hand.
Another spell used by Merlin to conjure a sphere of light on his hand was "Leoht" ("Light"). The young warlock cast it when he was in Drea's village, looking in the dark for the cause of the inhabitants' death. The sphere disappeared when the Dorocha approached, for Merlin could not use his magic near them (The Darkest Hour).