As someone said: Tartaros is the FT equalient of RM Blue Guardians,
- both are mass murderer
- both are considering humans as insects/tools
- as result a great is dead of the members by the end of the arc (Around the half of their members (Tartaros), all of the 6 Guards (BG) ).
So Alvarez is also the FT equalient of RM Four Demon Gods.
Given that the series has darkened and that it's a war, it's likely that all of the Spriggan 12 are dead at the end of the arc along with Zeref; and Mavis will be the one who helds Natsu alive, with Acnologia as the Arc Boss of the next arc.