This story is based on the fact that Tessa shows up at the end of City of Glass (if you don't believe me type "Cassie Clare Tessa in TMI" in google). It has also been revealed that Tessa took the place of an Iron Sister in Clary's Shadowhunter protection ceremony with Magnus.

"My dear, Tessa, what brings you here tonight?" I heard a voice from behind me. Despite the fact that I hadn't heard this voice in decades, I knew exactly who it was.

"Magnus Bane…" I laughed and turned toward the warlock. He was wearing clothing that reminded me of my teen years. In my mind's eye, I had glimpses of silver hair, a parasol with razor-tipped edges, a ruby necklace… "You know exactly what has happened to bring me here tonight."

Magnus touched the sleeve of my white dress, gently, as a brother would smooth his sister's curls. In a way, he was my brother. I felt a bit of pain in my heart as I thought of Nate. Nate…the betrayal stung even now. I brushed my curls out of my face. Then, Magnus turned slightly. To anyone else, it would've just looked like a change of position, but I could tell he was looking at a boy with black hair and blue eyes. "He looks very much like, Will," I said. "Do you like him?" I remembered how much Magnus had liked Will.

Magnus flushed at my comment. "You've definitely changed since we last met, Tessa. I remember there was a time where you'd kill yourself before you'd nose into others' business."

It was my turn to flush. "I apologize, Magnus. I did not mean to pry."

He laughed and turned back to me. "I really do not mind, dear Tessa. Yes, he looks very much like Will and yes, I've fallen in love with him. His name is Alec…Alec Lightwood."

I gasped. "Related to Cecy?" I could only guess by the last name and the darkness of his hair. He looked a lot like the Herondale family, though there were some features of Gabriel's in the boy.

"Of course, silly girl," Magnus laughed.

"You're much more jovial, Magnus. I'm so glad to see you've found someone who makes you happy," I commented.

Magnus didn't answer; instead, his eyes flicked upward, over my head. I turned around to see someone sitting on the steps of the Accords Hall. His face was in shadows. "Who is that, Magnus?"

Magnus was quiet for a few minutes. I had begun to think that he hadn't heard what I had asked. "That," he paused, probably just to make me even more curious, "is Jace Herondale." My eyes widened.

"Herondale?" I asked, incredulous. I looked even closer at the figure on the steps and I could see, in flashes of lights, Jace's face. He did not have the black hair or blue eyes of any of the Herondales I knew as a teenager, but there was still no doubt that they were related. There was a certain look about him, his cheekbones, his expression, even though he was deep in thought.

"Yes, he is one of your great-grandchildren," Magnus answered my unasked question.

"Oh," I sighed and looked up at Jace.

"He has fallen for Clarissa…Clary Morgenstern. Her mother is Jocelyn Fairchild. You remember them, right?" Magnus went on. I thought of Charlotte. "Actually, Clary is over there." I looked over where Magnus gestured and I met the eyes of a girl with green eyes and red hair. She had a similar look to Charlotte.

A strong sense of longing erupted in my chest. I thought of Aunt Harriet, Nate, Charlotte, Henry, Cecy, Gabriel, Gideon, even Jessie…but especially Will. I remembered something that Magnus had told me in the past, 'We are chained to this life by a chain of gold, and we dare not sever it for fear of what lies beyond the drop.' I had Jem now…but who was to know if or when the Silent Brothers' runes for immortality would run out…and he would leave me too.

Magnus touched my arm lightly and said, "I must go back to Alec. I also have to tell Clary where she can find Jace." I looked up into Magnus's bright cat's eyes, with a look of pain on my face. "Dear Tessa, you must get used to this. I'm sorry you lost your family, but it's been almost 150 years. Plus…some of these children are your descendants." I sighed and he continued with the farewell, "Don't be a stranger, Tess."

After he left, I walked off toward a figure off in the shadows…someone who looked almost 18, but was in fact, as old as myself. "Jem…" I said and cried into his shoulder.

"Shh, shh…my love. I know you miss him." It was times like these when I knew that Jem could be the same comfort Will was when Jem was sick all of those years ago.