Where is isaac? Pls no spoilers

So, I am watching Teen Wolf for the very first time. I am currently in S5.. But.. Where did Isaac went??

Spoiler!!:

After Allisons death.. I tought: Yeah, he needs time to process and will be seen again in a few episodes... bt he never came back?? Where is he??

Also.. Why wouldn't Derek help with the Death Doctors villians in S5?! Where is Derek? He just got like a superer uprgade of his powers after the kate fight a few ago,- and now he is not helping the suqad? I don't understand.

I am now in S5 Ep 9, maby i am to unpatient, but I my feeling, that isaac won't be back rises .. I don't get it.

Sry for grammer mistakes, english is not my first language. ♡