I think not only his magic, but Grogar to should have been trapped in the bell. The trio did the spell thinking they released his magic, but accidently set Grogar free. Chrysalis would not be willing to work for Grogar, so he traps her in the bell. Tirek and Cozy begin to help him
Grocord comes back and sees the real Grogar. A battle begins, but Grogar is to powerfull and traps Grocord too, who turns into Discord. Grogar goes to Canterlot to battle the protagonists. They managed to get the bell, but only Twilight can get away with the bell. Tirek takes the magic from the rest of the ponies with the help of Grogar.
Twilight sets Discord and Chrysalis free, and with the help of Starlight, Shining Armor and Cadence make a plan. Discords reveals his motive and tells them he was Sombra himself. He tells them Sombra can never be returned. Chrysalis is forced to help and discoveres the magic of friendship and reforms into a love bug. The windigo's appear during the planmaking.
The ponies of Equestria are put in Tartarus. Cozy gets betrayed because she let go of the bell, and Tirek takes her magic. Cozy goes to Twilight and tells to location of the ponies. Cozy goes north to escape punishment. Twilight frees them and together take on Grogar. Grogar has freed the Umbrum and Skeletponies, and together with the windigo's they are his army. He also brought back Sombra with scary voice.
The battle begins and Sombra is defeated by the pillars. At the end Grogar is trapped inside the bell, and Tirek and Sombra are turned to stone with the bell in the foot of the statue, unable to retrieve. Chrysalis is welcomed back into the hive and rules alongside Thorax.