dethz tuch sin is first mentioned in Chapter 27 when Tara and Profesor Sinister gave it to each other sadly when she found out that she had to 'sedouce' Volxemor. The second time, Darkness and Willow gave it to Ebony before she jumped sexily into the Pensive in Chapter 31. The third time, she performed it in greeting of the bastards in the band XBlakXTearX.
For all you posers and prepz out there, Tara means to say that Ebony gives them Death's Touch Sign, a totally normal, goffik thing to do. Now I really have to give Ms. Gilesbie credit she did not use the most common and most infuriating misspelling of 'sign' as 'sing'. But for God's sake! SIGN obviously has a 'g' in it.
Now that you know what the hell dethz tuch sin is supposed to say, what the fuck is it? The sign itself is done by placing one's middle finger of their right hand in the middle of their forehead while putting their ring and pointer fingers on their left and right eyelids respectively. The thumb and pinky are stretched out to the side. This was first found in the 5th book of the Saga of Darren Shan series (commonly called Cirque Du Freak) titled Trials of Death. The sign means that even in death you shall be triumphant (or I guess in this case triumelephant) and was used to wish Darren luck in his trials through Vampire Mountain.
Tara obviously uses it in Chapter 27 to compare Enoby's mission of seduction to the hazardous journey that Darren was forced to take. The second time I have no idea, everyone was just sitting around slitting their rists suicidally.
In Raven's story "I'm Not Okay", the character of Darren Shan actually appears as a Slytherin student (he is basically another goth, who makes a special request for blood during breakfast. The Mary Sue protagonist 'Eternity Dementia Johnson' drinks some of it, and loves it.), and thus so does death's touch sign. Raven tells readers, if they don't know what it is, to fuck off to a Britney Spears concert.