We have yet to know how Sombra returns to life, but from the leaks, it’s shown he is back and causing trouble, and since the Toy Expo had spoiled that Grogar was indeed the season premiere villian with the description:
 
“Grogar arrives from Tartarus to cause trouble.”
 
So until further info comes, I’m going with necromancy as the reason behind his revival.
 
And with the cliffhanger it pretty much confirms Cozy and Tierk are involved.
 
The chant is based on(with an extra line) the chant of Magica De Spell.
 
Twilight: You know Spike, I realized something. Sombra is coming back next season…
 
Spike: Sooooo?
 
Twilight: The comics are BS!