Bone Curse’s Sigil of Mortality works as written. It only applies Mark for Death on the targets for 2 seconds on cast. I have the node Cursed Limbs at 5/5 and thought maybe Mark for Death would be applied on hits then too but it doesn’t (I think this is intended though). Now, however, when I take Oppressive Gaze, then strangely enough I am applying Mark for Death on hits. I think this is unintended. Moreover, it looks like Mark for Death is not applied consistently through hits. Sometimes it works and other times it doesn’t. Note that I have no other means of applying Mark for Death as far as I am aware (e.g. no minions with the idol and no Haunting from Reaper Form).
Suggestion: I think being able to apply Mark for Death through hits with Bone Curse is nice but it makes Haunting from Reaper Form more obsolete. I would suggest that either Mark for Death is not applied through hits with Bone Curse or that it becomes an intended function (that then also works with Sigil of Mortality).