Stella was too completely focused and taken by her struggle against Twilight to possibly even notice, let alone react to, a blast that came too fast from too far. She only realised it was on her when it struck her, leaving a hole through her wing and into her chest. Her horn, previously about to touch Twilight's, leaned to the side as her whole body jerked in shock at being hit.
That moment of faltering had far worse consequences for her. She'd been locked in a struggle against Twilight, winning only slowly. No longer pressing against her, there was nothing to stop Twilight's side of the clash from rushing through. So it did, a stream of white magic running over her in full. Stella teleported out of it, but only after it had already hit her. Twilight, aware nothing was in the flow of her magic any longer, halted it. Her mane fell down and her body dropped to the ground, and she suddenly looked every bit as tired as she felt, barely standing.
Stella had gone to the ponies she'd imprisoned. She was going to kill them, at least some of them. Twilight had dropped her end of the deal, and there would be consequences. That, at least, had been her plan. When she tried to reach out with her magic towards the prisons she'd created, a moment after appearing there, she found they were refusing her. Before she could figure out what was happening, red lightning erupted from the spheres and struck her, pushing her back. She watched from the ground as the bubbles shattered in showers of red sparks, but before the ponies inside them could attack her they were teleported away in flashes of red.
Better for them, she thought as she was getting up. She would have mopped the floor with their guts, not hindered by her own refusal to use her coil against them as she was doing with Twilight. She turned back to the alicorn in question, greeted by the pitiful sight of a mare sweating and panting and looking at the ground as her body lightly shook. "You got your friends free. Good job. I suppose that's a fitting last thing to do before you die." Stella was charging a spell on her horn again. Twilight raised her head to look at her. Then she vanished.
Stella looked around in surprise, and that allowed her to actually see the next red blast as it came. She lowered her horn and released the spell she'd been holding, and the two clashed and exploded in the air, kicking up a cloud of dust and pulverized glass. Stella stood in wait as someone approached.
A figure walked forward through the dust as it settled. She took off her hat and teleported it away with her magic, then did the same with her cape. A black amulet hung around her neck, with a red gem in it, and the same red glow pulsed from her eyes.