Dark Souls Remastered | Be careful the two Mass of Souls in New Londo Ruins

There are two of them that appear in the lower area, which is only accessible after opening the floodgates. They never respawn the same playthrough after killed, like most enemies do when you either die or rest at a bonfire.

If you're not well equipped, these can kill you fast. Avoid the slime it throws at you and the spear that thrusts out from the mouth area.