When Terezi first lost all of her sight, she was all alone. She couldn't talk with anyone, so it took her sometime to find out that Aradia died. She learned all about it from Karkat, and she tried to comfort Sollux.
He never replied to her, and Terezi became very worried about him. One night though, a knock came from one of her windows. Still learning to walk around her hive, Terezi carefully goes over to the window. She smells someone floating outside and cautiously opens the window for this person.
As the wind blows inside, Terezi could smell who it was. The golden blood psionic traveled all the way to her hive. He's in pain. He's in a lot of pain.
"SOLLUX, 1S TH4T YOU?" asks Terezi, smelling the air more.
"hey tz," Sollux's voice cracks the slightest bit, as if he was crying.
"SOLLUX, H4V3 YOU B33N CRY1NG?" Terezi asks, worried about him.
"ii haven't," his voice cracks more, giving him away.
"COM3 1NS1D3, SOLLUX," Terezi moves out of the way for him to come in. Sollux reluctantly floats in, sitting on the window edge. She stands next to him, looking at his shoulder. Terezi doesn't know it is his shoulder, she thinks it's his head.
"tz, you're lookiing at my 2houlder," Sollux informs her.
"SORRY, 1'M ST1LL G3TT1NG US3D TO S331NG FROM SM3LL," Terezi acts as if he has been reading her messages.
"yeah ii read," Sollux stares at her with gentle eyes, "iim 2orry that you lost your eye2iight."
"1TS F1N3, DON'T WORRY 4BOUT 1T," she turns her head to face where his head actually is.
"can ii tell you 2omethiing, tz?"
WH4T 1S 1T?"
"iit2 about aa."
"T3LL M3 WH4T3V3R YOU W4NT TO T3LL M3," Terezi offers up her ears for him, and he begins to tell her everything.
It was really hard for Sollux to tell her about what Vriska made him do. Only part way through it, he began to cry, and Terezi offered up her shoulder for him. His golden tears soon drenched her shoulder, but she didn't even think about pulling away from him.
Terezi knew that Sollux needed someone there for him, so she learned about how painful it was for him to watch Aradia die by his own hands. It was Vriska's fault. Terezi told him many times that it was her fault. He understood fully that he would have never done that on his own accord, but he still felt really terrible about it.
Because of this, Karkat came up with a thought that the two were moirails, and the two of them never denied his claim. No one really believed Karkat. Terezi and Sollux did, though, decide to become moirails.