Logi appears by that name in Gylfaginning in the tale of Thor and Loki's
journey to the castle of the giant Útgarða-Loki in Jötunheimr where Loki
was pitted against Logi in an eating contest. The contestants appeared to
be equal in speed at eating meat from the bone, but Logi also consumed
the bones and even the wooden trencher in which the meat was placed.
Útgarða-Loki afterwards explained that Logi was really fire itself.
The Gylfaginning tells the story of Gylfi, a king of Sweden, who after
being tricked by one of the goddesses of the Æsir, wonders if all Æsir use
magic and tricks for their will to be done. This is why he journeys to
Asgard, but on the way he is tricked by the gods and arrives in some
other place, where he finds a great palace. Inside the palace he
encounters a man who asks Gylfi's name and so king Gylfi introduces
himself as Gangleri. Gangleri then is taken to the king of the palace and
comes upon three men; High, Just-As-High, and Third.
Gangleri is then challenged to show his wisdom by asking questions, as is
the custom in many Norse sagas. Each question made to High, Just-As-
High, and Third is about an aspect of the Norse mythology or its gods, and
also about the creation and destruction of the world (Ragnarök). In the
end all the palace and its people just vanish and Gylfi is left standing on
empty ground.