This weakaura only activates when you have the Serpentine Skuller (10 shadow resist wand) equipped since it's pretty likely you'll have that if you're tanking. If you don't use that wand then switch it to any other item that you only use while tanking (some shadow resist item probably).
The Salvation warner only loads for alliance players and it's a button that you can click to remove your salvation instead of having to right click at your buff bar and potentially right click some world buff/consumable on accident.
The Gift of Arthas warner is also a button that will use a Gift of Arthas when you click it.
The Demonic Sacrifice warner only appears if you have the Demonic Sacrifice talent.