When a deer logs out I perceive it as a sort of small death. It stops existing in The Forest. The deer's body and soul disappear into an alternate universe outside of TEF. In this state the deer is asleep in some sort of comatose, free from dimensions such as space and time. Nonexistant, yet still here as a part of something endless. This is until it returns to The Endless Forest, until the player logs in again and the deer gets "reborn".

Yet the longer the deer stays away from The Forest, the more instable the connection between deer and forest becomes. And as soon as the deer begins to lose connection, it starts to gradually become one with this alternate universe. First the body disappears, then the soul.

This is what happened to Mozart. He would stay away from The Forest as long as necessary. His body had disappeared, but his soul was still tied to TEF. He was still connected to it -- with the strings that tied him to his friends.
(I admit that strings are quite a cliché, but I thought taking them literally would make for something interesting)

I portrayed his soul by playing Phase 2 of TEF which gives your deer the appearance of a ghost. It's a pain to move and interact in it though so I sadly couldn't pull everything off as good as I wanted to.

Untying strings would follow in different stages.
Mozart first would avoid interaction with those who he still was connected to because he knew it would be the last time he would see them. After realizing he has no other choice to free his soul, he would approach them like he would do normally, albeit with very limited interaction, seeing as it is just a "part" of him still tied to The Forest.
Then he would drift back into a different dimension, but quickly return to the reality of The Endless Forest again. Now in this second visit as only a spirit he would be blind to other deer and call out for someone, trying to follow his own strings, but without any success.
Mozart would drift into another dimension again, and return into The Forest a second time. This time he would only see those deer he is connected with, and he would only be visible to them (unfortunately this can't be played out in-forest). Only now he would properly part with them and untie the string.

Unfortunately not everything went as planned. We could not manage to find Virgil in time and we could not say goodbye to him, so his string had to be torn.

Although Mozart never had the courage to tell, Dusk was more than just a friend to him. He was too afraid to show, but especially in those past months he had grown very dependant on him.


Ultimately it was the disappearance of friends, the lack of contact and the feeling of being forgotten that have led Mozart to such a decision.
I personally have been considering his death since a long time, but in the end everything was rather spontaneous and quick. The circumstances were right so we simply took the chance.
His death was meant to be quiet, nothing epical, that just does not not suit him.