Joshua 20:6

And he shall dwell in that city, until he stand before the congregation for judgment, and until the death of the high priest that shall be in those days: then shall the slayer return, and come unto his own city, and unto his own house, unto the city from whence he fled.

Cross-references

Joshua 20:9 reiterates the same purpose of refuge cities for unintentional killers, summarizing the rule in verse 6.

Numbers 35:24 describes the assembly judging between slayer and avenger, directly corresponding to the trial mentioned here.

Numbers 35:25 is the legal source: the manslayer remains until the high priest's death, which Joshua 20:6 implements.

Numbers 35:12 ensures the killer stands trial before death, which is the first part of the process described here (trial then high priest's death).