Board Thread:Show-Related Questions and Answers/@comment-5521796-20130304065819/@comment-5736616-20130304090407

Maybe it is meant to symbolise his revenge on Rumple for what he did to Milah, you know dagger through his heart and all that. It could serve as a reminder of what Rumple had done, we already know it is something he has been focused on and obsessed with for over 300 years. I'm guessing it found out about it from Neal sometime during his time in Netherland.