Return of Who, How, Why and from Where?

Basic question, we still get a big update called RoT. But who are we talking about and how they are coming, why they are coming and most importantly from where?
The only thing keep coming since update is strange creatures but all of them are old creatures (in lore) in world and new ones are "mutated" ones so that don't makes them a new creature that came from another place.

If you guys discussed this before you just can't leave link of thread and thread can be locked.