Can someone tell what is that weird note? Not the tie between the 12 and 12, afterwards there a slash line that I can't recognize and it sounds similar to a slide but not exactly.
It from "The Thrill is Gone" by BB king from the solo.
The slash is a slide (specifically a slide down) and the note you see is a half rest. It is just like any other slide except you just do not end your slide on a note you just rest instead. In this case you would start from the 12 fret and slide down and eventually rest.
A slide like this is just used for effect and there are slides that do the opposite where you don't start your slide on an specific note and slide up or down to a specific note.
Here's a site that show's more notation you may see in guitar tabs.