Death. Death is what separates us at the end. When your soul finally passes through Letherna in the Shadowfell and on to whatever passes as an afterlife for you, there are some ways of coming back from it.
Death isn't the end for a character, though it does typically mean that they are no longer on the material plane. When a character dies, there are a variety of things that could happen to their soul. They may go on to serve their deities, work in the workshops of Mechanus or join the Devils in their eternal Blood War with the demons. In any case, the DM should talk with the players about their character's fate, some character's may not wish to leave their afterlife, while others may not be able to.
If the player still wants their character brought back to life, they will have to make a Resurrection saving throw first.
To learn more about what might happen to characters, jump down to the Tendrils of Death section.
Resurrection Saving Throws
When you die, there is a DC put upon any resurrection magic to bring you back. This is the tendrils of death pulling your spirit into the afterlife and away from the material plane.
The Base DC is 10, and the target of the resurrection magic must roll a saving throw, called a resurrection saving throw, to determine if death clings to their souls or they are able to break free from the tendrils of death. Unlike other saving throws, this one isn't tied to any ability score.
Whenever you are subjected to resurrection magic, the DC for bringing you back from death increases per the table below. This happens after the spell has been cast, and whether or not it was a success.
Example: If you were targeted by a Revivify spell, and failed the roll against the DC, the next time a resurrection spell is attempted the DC is increased by 1, this increases the DC from 10 to 11.
If a creature is able to cast Wish, they can wish to bring a creature back to life. This does not require you to roll a resurrection saving throw and it does not increase the DC for future resurrection saving throws, though this can only be done once per creature.
Furthermore, a creature can instead cast Wish to reduce the DC of a target creature's resurrection saving throw by 4 points. This can be casted once per creature and brings the DC down to a minimum of DC 6.
Tendrils of Death
When you die, your soul leaves your mortal body and passes through Letherna to be counted by the Raven Queen. You are then whisked off to join the afterlife. This afterlife could be a variety of things, some good, some bad. In any case, the player should be aware what type of afterlife their character is in, some souls may not wish to come back.
A few examples are:
The Pious Souls
For the especially pious souls, they could find themselves on the beautiful shores of Hestavar, a plane watched over by Pelor, the god of the sun, and Erathis, the goddes of civilization and invention. There is also the Swan Tower, a white tower that is the home of Ioun, the goddess of knowledge. For more information on Hestavar, see Manual of the Planes (4E) page 94.
The Corrupted Souls
For the corrupted souls, they may never make it out of the Shadowfell and instead they become part of the Sorrowsworn, twisted creatures that become manifestations of raw emotions. For more information on Sorrowsworn, see Mordenkainen's Tome of Foes (5E) page 231.
Some souls may become cogs in the war effort between the Devils and Demons in the Blood War. By selling their souls to devils, or demons, they have no ability to come back to the material plane and must spend their afterlife fighting an eternal war.
All souls must pass through Letherna, and then are given to the gods they worshipped. If they worshipped multiple gods, they may be given a choice of where they wish to go, or a god may claim them faster than the others. Even if a character didn't worship a god, there is still a place for them. If they were an especially noteworthy thief, they may get claimed by trickery domain god or sent to some sort of prison if they stole from another god's chapel.
It is up to the DM to make the player's aware of what is happening to the character's soul, as their story might still continue on in the afterlife... or become an angry revenant on the material plane, hunting down the person that killed them.