He is a fictional character created by H. P. Lovecraft.
Some people treat Lovecraft's characters as thought/godforms.But what we do know of them is indeed fiction.Peter J. Carroll uses Azathoth in his chaos magic practice as a last ditch effort for when he needs something to change and only some kind of catastrophic action can fix it .Azathoth, in his writings, is the embodiment of uncontrolled chaos and destruction thats all i know about him
And for summoning If you believe in thought forms and godforms, then you'd summon/call/however as if he were a spirit or deity.Though I suppose they would be egregores.
They say H.P.Lovecraft got his stories from these alien beings visiting him in the dreams. After he wakes up from these dreadful nightmares he writes them down in a journel. Some of it is fiction and some of it isn't. The only way to know is to find out for yourself and actually throw yourself into these alien beings. I would say it's all encyrpted in someway but once you take your mind into the unknown you will find yourself these beings are actually real. Only the ones actually believe will know.
As most lovecraftian entities cause cataclysmic destruction and insanity, I would not attempt to summon them. A inter dimensional god of chaos is never needed. If you tried to summon Hastor for example, the stories say he will feed from your mind and kill you. Only a very powerful and skilled conjurer should undertake such a task. My screen name is a reference to Hastor Btw and even that was done with respect.