8 parts Complete Dagur's dragon, Sleuther, shows up at Dragon's edge, scared and riderless. Back at the base he seems to be slowly recovering, until his strange condition makes itself known. This takes place around the season 4-6 range. (warning for parasitism- sorta. This is the weirdest thing I've ever written, I'm so sorry.) I just had this idea after the twins talked about parasitism...
*please, for the love of God, if you know me do not read this D: You will never be able to look at me the same way ever again lol...* Also, I am the unfortunate owner of this, but not httyd.