Today i already beat medusa and eldyr boss 3 times in a row. They let me choose 1 costume from list so i asked my friend. After that i had a call so my game disconnect and i cant choose costume again ( none one in my bag too). 

Hello, thank you for reaching out and sorry for the inconvenience this may have caused.

 

Unfortunately, there is nothing we can do about it, because when you got disconnected, it closed the dialogue with the NPC, making you not able to get the reward. Things like this happen to everyone from time to time, and in the end, it's just a boss fight that you can revisit every once in a while.
Don't worry, the costumes are a random reward, so you still have a chance to get it every time you battle that same boss again.

 

This might not be the response you were hoping to get, and I apologize for that. Since this is resolved, I'm gonna lock this topic now. Thank you for your patience and understanding.

Kind regards, Thaloras.