Disclaimer: I am not Cassandra Clare.
It took Magnus three hours to summon the demon. This surprised everyone because that was extraordinarily slow. Eventually, though, Magnus explained that while summoning the demon he had to also partially transform it so he could control it in 'speculo'. It had been Jace's idea to use a Marbas demon "because those are relatively easy to kill" he had said.
Now that demon stood before them trapped in the pentagram.
"Could we just leave it there for a bit?" asked Magnus, "My magical reserves are entirely drained."
"Well," said Jace, "if your magical reserves are drained, then we don't have much choice."
"I think we could all use a rest." Mayrse had come up behind them.
As everyone dispersed Clary went over to Mayrse.
"What have been doing all this time?" she asked, attempting casual conversation.
"Trying to fix the desk." she answered.
"We can always buy a new one you know."
"I know... It was such a wonderful desk though..." she said wistfully, "Not," she added quickly, "that it really matters, now that we've met an angel and so on."
"Shouldn't we tell the Clave about what the said angel said at some point?" Clary asked.
"I hate to keep the Clave in the dark about something so important," said Mayrse, "but they could interfere and now that we have everything started we might as well wait in till we have the mirror to say anything."
"I see." answered Clary, before heading off to her room; she might as well take advantage of her free time, after all, she hadn't drawn anything in days.
An hour later Jace bounded into her room causing Clary to jump up from where she was sitting in surprise, banging her knee in the process.
"Ouch." she said, and then: "Is Magnus ready to continue?"
"Yeah." was all Jace said.
They hurried to the library, Clary limping because if her knee.
"Hello! Hello!" exclaimed Magnus, his exhaustion evidently gone.
"Hello indeed." replied Jace.
"Let's get started." It was Isabelle, who hadn't said much since the angel had arrived, "We only have so long before the Clave starts asking questions."
"Right." said Magnus. He then went around the pentagram, where the Marbas demon still stood looking at them with hostility, touching each of its five points. At his touch each one began to give off a pulsing light. When he had touched all five then each let out a ray of lights that met the other rays in the center of the pentagram. Where they collided a large bright light pulsed.
"Woah." Clary breathed.
Then Magnus stood in front of the demon and began speaking in some sort of demonic language. The demon starred blankly at him. Then Magnus started muttering something, some sort of warlock language probably, and the light if the pentagram grew brighter and brighter in till it became blinding.
For a while everyone just watched while Magnus walked around the pentagram muttering, but eventually, one by one, the room emptied in till it was just Clary.
She just stood there one arm over her eyes, as not to be blinded. After a while Jace came and showed her out. Then she went back to her room; back to her drawing.
But she couldn't concentrate; the drawing refused to work. Maybe it was the anticipation, maybe it was something else...
After two hours Isabelle came to get her.
"Magnus is finished." she said.
"Ok." answered Clary, putting away her sketchbook and rising to join her.
They headed to the living room where everyone was waiting at the door. Magnus stood near the pentagram, looking even more exhausted then how he had been after he'd summoned the demon.
"Well," he said, attempting enthusiasm, "come and see. It doesn't bite."