gravelords... how do they work?

i have the gargoyles killed rang both bells and i Host FC matches on Sen's roof.

i was invading some undead parish right out side the gargoyle boss (which is defeated) while waiting to invade i decide to go kill some mobs, then out of no where a BP comes flying behind me and backstabs me and kills me.

i die, res, and as i am running back to my body to my surprise there are BPs EVERYWHERE. there is a big iron giant BP by where Ria kneels in the chapel, that was def not there when i was waiting to invade before i died. i never got the grave lord message and that area boss is dead.

i am not complaining it was cool as hell first time i ever saw them, but wondering why i saw them.

https://soulswiki.forumotion.com/t3296-gravelord-instructions-how-to-properly-spread-misery

Please remember to check the covenant section for stuff like this guys, its generally much faster than asking in general or pvp, as much of the information is already posted.