Lets say i lose my chance at darkria or maybe a later edition legendary for example mewtwo can the quest be added again?

Lets say i lose my chance at darkria or maybe a later edition legendary for example mewtwo can the quest be added again?

 

Hi n9ne1987, Horon, Thief of Souls was recently added to Dark Valley. This miniboss serves as the reset point for Dark Realm (for those who left it) and also serves to restore Darkrai if you have failed to catch it for any reason. You need to defeat him without any of your pokemon fainting. You can also obtain a halloween rapidash mount as a reward for beating him. A guide can be found here. I have not tried the method in the guide so cannot vouch for it but it'll give you a basic guideline of what to expect.

 

As for other catachable legendaries there is a boss called Nikola in Unknown place that you can battle if you fail to catch any of the catchable legendaries for a chance to recatch it. This link goes to the wiki page for Nikola boss and will tell you his line up and what legendaries you can recatch after beating him.