Dreams can be employed in magic in a number of ways. They were used in several ways in the Revival of English Magic.
At the Shadow House in July of 1809, Jonathan Strange successfully summoned Miss Absalom into a dream to request her aid in learning magic, using his own adaptation of Ormskirk's spell of summoning to ensure that Miss Absalom and he arrived in the same dream at the same time. Unfortunately, the presence of such powerful magic had a strong effect on John Segundus, who happened to be visiting the Shadow House with Mr Honeyfoot at the time. He fell asleep in the grounds and slipped into the same dream as Mr Strange and Miss Absalom, disturbing her such that she left the dream .
Magicians can also enter other people's dreams to investigate their content. In the Peninsular War, Jonathan Strange used this magic to enter the dreams of a Neapolitan company that had stolen six cannon from the French, in the hopes that he could find some clue as to where the company was. Unfortunately, he was able to find no information of use in the dreams, which were focused largely upon the duplicity of world leaders and on food – namely, a leg of lamb and some hard-boiled eggs that were abusing the unfortunate (and hungry) dreamer maliciously .
In the traditional tale of John Uskglass and the Cumbrian charcoal-burner, the King is said to have used that magic himself, when he employed his ravens to search the dreams of the charcoal-burner for evidence of magical ability [LoGA].
Finally, a magician can also send a dream to another person. Both Mr Norrell and Mr Strange used this magic, sending disturbing dreams to Napoleon Buonaparte and Alexander I, respectively, in order to hamper their actions. Mr Strange had considerable more success in this. Mr Norrell's dream-spells themselves entered Buonaparte's mind perfectly, of course, but he was, alas, unable to create a dream that was at all frightening .