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).
The potion Morgana poured in Gwen's wine had the effect of a guiding light: when Morgause chanted a spell ("Beo þu leohte bewunden"), the path that the maid had followed was illuminated. After a certain time the potion began to lose its power and Morgause, who had followed Gwen with her men, had to chant another spell ("Scrin scir") to reveal Gwen's way (The Coming of Arthur: Part Two).
In "The Castle of Fyrien", Morgause enchanted her ring and gave it to Morgana with the spell "Lære us, forþbrenge us, ætlæde us. Wisu heofoncandel ure". When Morgana dropped it on the ground Morgause's
A guiding spell on Morgause's ring.
magic lit the ring and it produced a high column of yellow smoke.
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).