I like to think that Grogar’s bell can absorb not only magic from living creatures, but magic from artifacts as well, and even rainbow magic. Chrysalis, Tirek, and Cozy Glow turned everypony against each other (because nopony bothered to question anything. In real life, it would not be that easy to turn people against each other.), stole the magic of every creature more powerful than them, destroyed Canterlot, and pretty much undid everything Twilight and her friends had ever done. They won every single battle in that episode and came closer to winning than any other villain in MLP ever has (though Tirek came just as close on his own in season 4). Even when all of Equestria was united they still stood no chance against them. All they had to do was use the bell on Twilight’s giant rainbow laser. Had that worked, they would have won, every creature would have bowed down to their new rulers after witnessing Twilight’s death, and anyone who resisted would have either been imprisoned or executed, committed suicide, or would have given up and accepted defeat. They only had to do one simple thing and they couldn’t even do that (now had the bell failed they still would have lost).
And we all remember the only reason they were even a threat to begin with was because Discord gave them all that power. Had Discord never assisted Tirek at all, nobody would have ever heard from him. Season 9 would have been better off if Grogar (the real Grogar, not Discord impersonating him) was the main antagonist instead of those three idiots.