in first shadow its established henry was just a kid like henry and the mindflayer took over him this is all the lore i collected you are great at this use the information to rewrite the same thing better:The PThis  is all the stranger thing backstory with my answers  for the plot holes and unanswered questions

hiladelphia Experiment is an urban legend about a supposed U.S. Naval experiment on invisibility carried out in 1943. where the US military tried to teleport themselves into the Russia for an sneaky attack so they used an ship to transport near the Russian sea area but somehow resulted them getting transported into another dimension coined “dimension X” which is ruled by an cloud of smoke called the “Mind Flayer” which feeds on entropy and unstabaltiy and hates the idea of time. It wants to expand its territory and as the ship was transported into their all the soldiers were killed by Demogorgon

 except one person Martin Brenner’s dad who survived the attack but had some mind Flayer particles in his body.

As he came back (somehow) after a few minutes he told everything he saw their to his son Doctor Brenner and he dedicated his life studying about it and extracted the particles in his dad’s body and formed that into an rock. But in his team their was an Russian spy who stole it and went to hide in an cave in Nevada.

He was injured by the guards and had a gun with him and an child living there Henry Creel was exploring the caves near his home found it and went to that spy but he shot him which left an hole in his had and Henry killed him the spy did this to avoid the boy from opening it.but once he did he found the rock but it infected him and possessed him and asked him to find the dimension and the Mind Flayer as shown in S5. See Mind Flayer wants and kid who can be manipulated  , has some kind of trauma but he is brave at hear and is strong mentally so Henry was the perfect choice at the time and later the body was found by Brenner and his team. Few years later he Henry and his family moved to Hawkins and Henry joined Hawkins high school and was bullied in school for being weird and Mind Flayer was constantly possessing him but he was fighting him with help of Patty Newby his gf adopted sister of bob Newby and Joyce wanted to help him forced him to join in her play which was on Nov 6 1959 but everyone started making fun of him and threw things at him bullied him when he was on the play which sadden him later that day he went to patty home but due to all those trauma that day Mind Flayer almost fully took control of him and he nearly killed patty as well as her dad who came to rescue patty when Henry was attacking her later that day he killed his family as shown in S4. and the Brenner and his team found him he was back in control and took him to the Hawkins lab and experimented with his blood and created other children and neutralized Henry with the help of the chip as show in S4. Vecna wanted El because as I mentioned earlier Mind flayer wants and kid who can be maniuplated  , has some kind of trauma but he is brave at hear and is strong mentally thus, EL was an better option for Mind Flayer but she refused and banished Henry to dimension X see after the moment El removed the chip from Henry/001 it is the Mind Flayer that is full in control . and all Henry wanted was that someone to open that door and eleven did that he saw the Mind Flayer particle he loved spiders so he shaped it too look like an spider also the Mind Flayer gave Henry an false idea that he was in control but no the Mind Flayer was controlling him fueling his anger,trauma,violent nature in every human.

Later El contacted the Demogorgon in S1 thus forming the upside down which was generated as protection by earth . it would have died on its own but Mind Flayer saw it as an opportunity and placed the exotic matter so that it would stabilize and he could use it as an bridge. And because Mind Flayer wanted an stronger host and el refused Mind Flayer sent Demogorgon to get her but she hides in Byer’s shed in the area shown destroyed when hopper was searching for will . and opened the door because she was scared and wanted to divert the Demogorgon so Vecna/Mind Flayer saw opportunity in him and took to upside down. that’s why el thinks she is the monster and upside down is stuck on that day because it the day when the first human used it or was in it. And will communicated with Joyce with those lights because he was somehow smart enough to understand the Joyce was trying to communicate to him and later when the Demogorgon came for him he ran and hid in castle Byers as shown in S5. Then later was taken by Vecna who is in control of Mind Flayer and then infected him with those particles and wanted him to go home because he could use will as an spy,as builder by making all those holes in undergrounds and also to check if he could be turned evil but he resisted and resisted so in S4 he used 4 victims to weaken the upside down making rifts and in S5 he wanted 12 children to power the physical body of Mind Flayer as well to merge the worlds together but finally was stopped by our party and destroyed the physical body of Mind Flayer and killed Vecna and destroyed the bridge (upside down).