"Welcome to the murder bridge!"

Ive heard it a few times...before/around halloween. Never does it multiple games in a row

I've only heard it once, and I've played quite a few arams, so I assume its an easter egg? Not sure honestly. I think I heard it about a month ago.

So I heard that announcement on ARAM even though the text said "Howling Abyss" is it an easter egg? or new?

It is an easter egg, but it's not new. It just has a low chance of playing instead of the normal sound clip.

It is an easter egg, if you listen closely to the shop keepers they will reference their regrets on not naming the Howling Abyss, The Murder Bridge. I'm assuming that "The Murder Bridge" was either Rito's code name for the Howling Abyss while it was in development, or it was just a possible name they wanted to use.