Why does Tokoyami's quirk Dark Shadow goes out of control? In the most recent episode of Boku No Hero Academia, we see that Tokoyami's quirk Dark Shadow loses control. What is the reason behind that and what exactly is Dark Shadow?
I think they explained it well in that episode. When it's in the dark, dark shadow gets stronger but it's harder to control. The dark plus Tokoyami negative emotions contributed to dark shadow growing stronger and getting out of control. Light in the other hand, makes it weaker and easier to control. It's also mentioned in this wikia


  According to Mezo, Fumikage's negative emotions (such as regret and indignation) probably intensify Dark Shadow, making it more unruly and possibly making Dark Shadow stronger as a result, albeit at the cost of Fumikage losing control; Fumikage himself stated that his anger causes him to lose control of Dark Shadow.


http://bokunoheroacademia.wikia.com/wiki/Fumikage_Tokoyami