i am getting only three pokemon to fight in quest. Also in previous quest for catching darkrai, rattata didn't join party even after finding all his friends. I ignore those things and completed quest but now in umbra hard quest i am facing problem due  to last quest's bug.

Greetings @Gawarim

 

Apologies for the inconvenience 

 

The pokemon that are available to you in the Nightmare Battlefield will be those that were recruited. Since you did not recruited rattata during the Dark realm, you will have to return there using Horon. Here's how you need to proceed : 

 

1. Beat Horon in the dark valley with no fainted pokemon to reset the dark realm. If he gives you an Halloween mount, you will have to wait for 3 days for him to re-appear and repeat this step

2. Beat Alucard in his castle, on the third floor, with a pokemon with the move miracle eye in your party 

3. Return to the Dark realm and complete it. Recruit all the possible pokemon and beat umbra, the mega gengar (This step is mandatory to re-access the Nightmare battle field.)

 

You can also refer to this guide if you are ever lost. You can find all the rattata friend(s) using the picture below : 

Spoiler

 

4. Re-access the Nightmare battlefield in the shrine and you will have all the pokemon's you recruited available to you 

 

Let me know if that works! 

 

Kind regards

Midsommar