From Destinypedia, the Destiny wiki
Darkness Zones are areas that Guardians encounter that separate public and private spaces. Guardians starting a story mission or strike will begin in a public area, and may encounter other guardians. However, once they enter a darkness zone, which is only accessible during that mission, they will be separated from randomly encountered guardians. Guardians that are part of the player's fireteam will come with them. 
Entering a Darkness zone is especially hazardous; not only are players separated from anyone outside of their fireteam, but respawning is also restricted. If a solo player is killed, or an entire fireteam has been wiped, Darkness consumes the player(s) and they will have to start over at the nearest checkpoint. However, so long as one member of a fireteam is still alive, it is possible to revive and respawn players. In Destiny 2, the same restrictions apply for the most part, except Normal and Prestige difficulty Raids. In Destiny 2, the Darkness Zones are called "Restriction Zones" probably due to the base game not mentioning the Darkness.
Respawn restrictions depends on what situation the player is in.
Note that Arena challenges have the same restrictions as normal mode Raids.
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