Minecraft – Teleport Player From Void To Spawn

I don't quite understand what you mean but there were 2 things I thought you could have meant.

The first one was to spawn the Mob with half a heart so there a one-shot-kill and for that, you would do: /summon Zombie ~ ~ ~ {Attributes:[{Name:generic.maxHealth,Base:0.5}]}

The second thing I got was to make the Mob Invulnerable and for that, you would do: /summon Zombie ~ ~ ~ {Invulnerable:1}

However, if you're trying to spawn a mob that has an attribute to define "Hit by player", unfortunately there is no such command.

First, you should set up a scoreboard objective of type stat.leaveGame:

/scoreboard objectives add JustLeft stat.leaveGame

When a player leaves the world, their JustLeft score will increase by 1 because of its type.

On a clock somewhere, you should then have:

/scoreboard players set @a[score_JustLeft_min=1] Death 1

To set the death score of players who have just relogged to 1. Followed by:

/scoreboard players set @a[score_Death_min=1] JustLeft 0

So that their JustLeft score is reset after their Death score is set, preventing them being continually killed.

@a only selects online players, so they should only have their Death score set and then reset when they rejoin the world, even though their JustLeft stat will increase to 1 straight away.