Azazel. Again.

Yes, yes. He can be totally crazy sometimes. But it has to be said, that just as Marco Aurelio told him, the swap-body experience empowered him, greatly. As soon as he started to feel better, he realized it. He can use magic in an even easier and more fluid way, can hold on longer while using it, even more energy-demanding spells are being handled much better by his body.

The process was: while he was in bodies with no magic, his mind kept pushing the body to try to awaken some magic, without even him to realize it, it was like a natural reaction of someone meant to have magic within. It worked like heavy training and once he was back to his body, his whole perception and control over his magic was improved, boosted.

That reason and the fact he got his hands on a very ancient book, gave him the courage to consider the option to summon Azazel again, in his apartment this time.
Why him, of all the Greater Demons? Because Azazel is known to be the Forger of Weapon, the demon who taught humanity how to craft weapons. Since the Iron Sisters said they could create no such weapon to separate Jace and Sebastian or harm just one of the two and that the only chance could be an Archangel Sword, Magnus thought that maybe, Azazel could be able to create something himself or to help them in other ways. This time, he knows he can contain him, also since the most part of his essence is imprisoned in Dudael again and only his spirit can reach them.

That's what the candles were for. Magnus prepared all the pentagram, applying all the candles all over the living room, with the help of the other guys as they really need many.
When everybody is ready and all the candles are lit, Magnus starts with the formula, a much different one this time:

•Quod tumeraris: per Jehovam, Gehennam, et consecratam aquam quam nunc spargo, signumque crucis quod
nunc facio, et per vota nostra, ipse nunc surgat nobis dicatus Azazel!•