Lullabies for Time Tots

From Tardis Wiki, the free Doctor Who reference

Lullabies for Time Tots was written by the Benncuiq III. (PROSE: When It Was Fun) It told the story of the Death Zone and the Tomb of Rassilon. (TV: The Five Doctors) It went,

Who unto Rassilon's Tower would go
Must choose - Above, Between, Below
Who unto Rassilon's Tower would go
Must fight with those he used to know
Who unto Rassilon's Tower would dare
Must dance across the deadly square
If Rassilon's Tower you step in
Then you must choose - to lose or win

At some point it came into River Song's possession while she was researching the Fifth Doctor. (PROSE: When It Was Fun)