From Tar Valon Library
When the group that had left the Two Rivers was forced to seek refuge in Shadar Logoth from hordes of Shadowspawn, Mat, Rand and Perrin went out alone to explore the city . Whilst doing so, they encountered Mordeth, who had hoped to accompany them to the edge of the city and then possess one of them, thus escaping his imprisonment there. Instead, he became angered when he learned they were going to Tar Valon and when Rand realised he had no shadow, he attacked them and they were forced to flee (TEotW, Ch. 19). They escaped, but Mat took with him a curved dagger with a gold scabbard and a hilt capped by a large ruby (TEotW, Ch. 20). When they told Moiraine of this, rather than asking if they had taken anything, she asked if Mordeth had given them anything, to which Mat was able to say no (TEotW, Ch. 19). As such, he left the city without her being aware of it.
Over the next few weeks, he became tainted with the suspicion and mistrust that had destroyed Aridhol (TEotW, Ch. 20; Ch. 26; Ch. 31). By the time Moiraine found him at the Queen's Blessing in Caemlyn, he was refusing to see anyone but Rand and tried to attack her with the dagger when she came close (TEotW, Ch. 41). She was able to temporarily Heal him, but said he would need to go to Tar Valon to be fully healed (TEotW, Ch. 42). Until then, he needed to be close to it, or would grow sick. They were diverted from this by hearing of a threat to the Eye of the World (TEotW, Ch. 42) and ended up in Shienar, where Fain stole it along with the Horn (TGH, Ch. 6). As Mat needed it for the link to be severed, he was forced to travel after Fain to try and recover it (TGH, Ch. 7). The three boys tracked it to Toman Head, where they retrieved it and the Horn (TGH, Ch. 47).
Mat and the dagger were taken to Tar Valon, where a circle of ten Aes Sedai healed him using a sa'angreal (TFoH, Ch. 10. He was then free of it, but the dagger was considered so dangerous it was kept in a locked storeroom, deep in the Tower. Fain was able to sense it and managed to steal it, alerting an Accepted, who he killed with it (TFoH, Ch. 19). He kept it with him and later attacked Rand with it (ACoS, Ch. 36), Rand's life only being saved by the quick actions of Samitsu, and later Damer (ACoS, Ch. 36) and Corele (ACoS, Ch. 41). Fain escaped and kept the dagger with him, later killing Kisman with it (WH, Ch. 22).
A curved dagger with a gold scabbard worked in strange symbols. Fine gold wire wrapped the hilt, which was capped by a ruby as big as Rand's thumbnail, and the quillons were golden-scaled serpents baring their fangs (The Eye of the World, Chapter 20).