So i played with a friend in a Aternos server in 1.16.5, with the latext version of mod, we logged off and come back some hours later, the boss just....disappear with no trace. where we were we can see him but now there's nothing. Did i need to do something ?

Shouldn't have disappeared. If you do "/witherstormmod chunks get" while in the overworld, what does it say?

it do nothing, the wither is gone.

May be something with aternos however I'll take a look

Shouldn't have disappeared. If you do "/witherstormmod chunks get" while in the overworld, what does it say?

This happened to me in singleplayer in 1.18.2. I wrote the command and it says this "Wither Storm mod has 0 chukns out of 0 forced chunk loaded" I just left the nether and it's gone

happened to me to i was about to go inside it i slept and it was gone

May be something with aternos however I'll take a look

Thank you, btw you're mod is really cool and if you have free time, you can try to add some "details" to it.
And is there a command to have the Wither Storm grow faster ?

Unable to reproduce. If you could find a way to reliably reproduce the issue that would be great. You can use the "/witherstormmod evolutionSpeed" command and set it below 1 to make it evolve quicker