I need a brilliant excuse to get out of a nightmarish evening

It's quite simple. Say "I'm not coming". Period. End of story. next?

How about:
You: "I'm sorry, something's come up and it looks like I can't make it to the party."
Them: "BUT WHY OH WHY NOT GOD WHY NOT!!!"*
You: "I'm sorry to disappoint everyone, but I just can't make it."
You: walk away.

*sorry, but this is how I imagine your office reacting. Why is it so imperative that you go?

Clearly Nom Deplume is actually SANTA.