Can I get back into Oryx's chamber if I died during the fight?
Solution 1:
When you die you die. If you notice, there are many different instances of the realm from the starting area. Before the battle with Oryx begins, he will tell you that the instance is locked "for you all to die alone." Unfortunately, this means you can not get back into that instance until Oryx is cleared. If you spend about 20-30 minutes in an instance though, you will end up at Oryx again before you know it. Generally in the time it takes to get a character to 20, you should see Oryx at least once.
And just some advice, I'm a new player as well, but I pretty much always die at Oryx if I try to fight him with the others. Until you can get some good gear and maybe stat potions on a character you like, you would probably benefit to warp to nexus once you get to the actual battle with Oryx. You can be one/two hitted even at level 20 if you are not extremely careful. You will still gain 2-4 levels on the way to fight him, but generally you won't get XP at the boss battle, nor will you get any drops if you hang around the edges just trying to survive. It's not really worth the risk of dying to stick around.
Solution 2:
You cannot return to Oryx's castle. If you die, you die, as turbo said. If you want to get a drop then, and are under level 15, if you are quite good then you can maybe risk attacking, but if you are not then I recommend that you stay out of Oryx's way.
When I was playing, I died because I went too close to him so he shot something at me. I wanted to return and destroy him but no one answered. Now I still die as a pro because my decisions are foolish.
If you want to get drop, you must do 2000 damage to Oryx with at least a t8 weapon. The way you damage hims is by going past his artifacts, without dying, and attack him while dodging. His artifacts are the yellow things that come out of him. Watch out for his minions, and, when he gets big, try to get as close as you can without being hit.
By the way, you can get to Oryx's castle if you complete an event. Someone must kill grand sphinx, cube god, skull shrine, and pentaract. After that in chat list Oryx will say that he has closed the realm so no one may enter it. After 2 minutes, he will cause the earthquake, sending the players to his castle. When all the players have gone to the castle the realm will open. If you are in a dungeon when this happens, you will not be transported to Oryx's castle.