After the events of the later British Lostbelt, you (for some reason) felt it was a good idea to bring Oberon to Chaldea.

The moment he materialized he was acting like himself, being sarcastic half the time. Pretending nothing happened. Standard fare.

But over time, things began to change.

As the weeks go by and he grows stronger, the Servant that you summoned began to change its appearance, revealing his silver hair. His crown has become a light blue. Finally, his gossamer dragonfly wings emerge behind him.

"Very well, Master... Did you really want to see this form again?" His blue eyes stare at you, masking his true feelings with hate and disgust. "Now you have it... Are you happy now? You're such a pain in the ass, seriously."