This post contains relevant info pertaining to both characters, that you may find helpful.

Grogar: not a necromancer in G4, though you could obviously still write him as one. He is the "Father of Monsters", known for creating many horrific beasts that spread havoc and fear. He existed in Equestria pre-unification, terrorizing the ponies with his abominations. Much of his magic was within his Bewitching Bell, which could absorb magic from other creatures and add it to his own. He was confronted eventually by Gusty the Great and her Unicorn Army. His beasts and the Uniarmy fought; Gusty was eventually able to steal Grogar's Bell, and seal it atop Mt. Everhoof. This weakened him dramatically, though it remains unclear if he was truly defeated, or simply went into hiding. He has not been seen since, either way.

Gusty the Great: a unicorn mare with little known about her past. She drew up the courage to finally confront Grogar, gathering an army of unicorns to combat his monsters. She was able to steal his Bewitching Bell and seal it on top of Mt. Everhoof. Her fate and whereabouts after this are unknown.

Bewitching Bell: An artifact of immense danger and power. It is capable of absorbing magic from other beings and transferring it to the user. This Bell contained much of Grogar's might, and was able to retain it even after it was sealed atop Mt. Everhoof for countless millennia. The magic it contained was enough to turn Cozy Glow into an alicorn, Chrysalis into a "super" form, and Tirek from his 2nd to 3rd form- all while still barely scratching the surface. Unlike Tirek's draining ability, it does not simply give the raw power of drained users; instead, it gives the full magic abilities. This is why Cozy Glow turned gigantic and gained uncontrollable chaos powers after using the Bell, when it had recently drained Discord.