dnd 5e – How can I kill a tough Druidian NPC outside the scene for the plot?

In a campaign I made myself, there's an 8th-level Druid NPC (good guy) who has an item that I want the bad guys to steal from him (killing him the way to kill him) dramatize).

My main concern is that he can easily turn into a bird and fly away (with the object).

There should be a witness who informs the players that the bad guys took the item. His defeat should be credible.

Here is the question:
Should I say as GM that the druid is dead and ignores all resources that a character of this level needs to defend himself?

Or should I, on the other hand, make an encounter (offscene) with a detailed composition of the evil forces to defeat him?

How could I work out this event?