The Teyn lighthouse quest (TR_m4_T_Lighthouse) is supposed to end once you kill the lighthouse keeper Erethys Omavel (tr_m4_erethys_omavel) by activating journal stage 100. However, to my limited CS sleuthing abilities, it seems that a script to activate Journal 100 upon his death was never added.

This was also reported by Revane during the DoD playtesting push, but it was never fixed: https://www.tamriel-rebuilt.org/comment/26017#comment-26017