Hello Michi! May you explain what’s the meaning of omedeath? Is there a pun behind it

makyun:

Oohh, omedeath is pronounced as omedesu in Japanese. Omedesu is the shortcut for omedetou desu which is like ‘congrats’ in English.

So Ishida-sensei was greeting Shirazu a happy birthday except he used ‘death’ instead of ‘desu’.