HELP

Hi all, so I moved house (slightly out of town) and my cat was fine and well adjusted for the first 3 weeks, after this he went missing for around 6 weeks. In this time we presumed him dead until he was posted on social media as lurking around our old neighbourhood for the last 3 days and I picked him up and brought him today. While he was missing we got a new kitten as a replacement. He has always been a meaner cat but still seemed to love our family, since we brought him home he has been acting extremely vicious - hissing at us and attacking when we try to touch him. I think it adds insult that there is a new kitten in his 'territory', and He tries to attack the kitten. My question is: how do I put him at ease with being home and will he return back to normal? How do I form a relationship between the two cats?

Can you separate them for days or even weeks?  That's very important when introducing cats to each other. They should get used to each other's smell first.  Blankets they have slept on should be exchanged.  They should have a lengthy chance to sniff at each other under the door instead of being confronted face to face.  Give them their treats at the door, so they begin to form positive associations with each other.  When the hissing stops and they get relaxed then you can try again to see if they can share the whole house.  And the kitten will be a little bigger and older and better able to handle the older cat if it does whap-whap-whap at the younger.

You wouldn't like it if you were away for awhile and came home to find a "replacement" and cats are actually more territorial than we could ever be.  How old is your cat?  

And finally, some cats just don't want to be around other cats.  Each is an individual.  We had to rehome a dog once who couldn't settle things with the dog we had first. They fought and fought for weeks on end and it got pretty serious sometimes where even full blast from the hose wouldn't drive the new dog away from the older dog.  Sooner or later there was going to be a huge vet bill.  Their personalities were just never going to be compatible and we finally had to admit it wasn't going to work.  The new dog was gorgeously beautiful and we didn't have trouble finding a new home. (I was secretly glad she was gone as I never got over being a little afraid of her. She was just unpredictable. And our other dog turned out to be a one-dog type of dog. Didn't get along with others even if they were a lot more submissive than that first one.)

If they aren't completely incompatible, going slower with the introduction should help a lot.  Neuter both of them, please?  that will help a lot if it hasn't already happened.  They'll be less territorial and more easy-going.

SilkyM is right.. you really do need to do careful introductions.. with the emphasis on patience!

If you follow the instructions in this article, you should do well.

And welcome to TCS...