Banner
Every 50th kill of this enemy drops Zombie Banner.where it turns up
It only appears inside a Graveyard, so in practice you meet it wherever enough tombstones have piled up, whether you built the biome deliberately or just kept dying in the same spot. Everything about how it walks and attacks reads as an ordinary zombie until the moment you kill it.
the death trigger
Killing one is not the end of the encounter. Each one bursts into a small cluster of maggots on the way down. Against a single zombie that is a non issue. When a graveyard has pushed several onto you at once it becomes a floor full of grubs while more zombies keep walking in, and clearing them takes attention you would rather spend on what is coming next. Pull them a few at a time near a doorway or a ledge you can back onto, and sweep up the leftovers before you go get the next group.
banners
There is no banner of its own, despite how differently it dies. The plain Zombie Banner already covers it, so if you have one hung up in your graveyard you are already getting the bonus.