Ride the dragon
You climb from it’s grip to it’s back and start blasting it with every combat spell you have, but the dragon seems uneffected. Usually celestial beings are vulnerable to elemental magic, but it looks like the undead part of it is dominant.
What do you do?
Holy Magic!
Go on a magical adventure on your new friend.
OR
Stab it.
make this dragon your familiar using some sort of creature control spell.
Use healing magic on it and see what happens.
Tame the dragon and turn it against its former dragon buddies.
dont mind the other comment, that was my brother.
heal the dragon
Become highly feared as people watch you ride the dragon as it kills millions… even though you can’t control it.
Heal it.
Oops. Guess that was suggested already.
Make friends
you are a cleric. do what you do best.
Wait for it to land.
Take a moment to enjoy yourself. You’re riding a dragon, afterall.
Channel the mighty power of your deity to Rebuke Undead!
Actually, scratch that. You could get hurt. You don’t want that. xD